# Machine Learning Model Evaluation

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If we were to list the technologies that have revolutionized and changed our lives for the better, then Machine Learning will occupy the top spot. This cutting-edge technology is used in a wide variety of applications in day-to-day life. ML has become an integral component in most of the industries like Healthcare, Software, Manufacturing, Business and aims to solve many complex problems while reducing human effort and dependency. This it does by accurately predicting solutions for problems and various applications.

Generally there are two important stages in machine learning. They are Training & Evaluation of the model. Initially we take a dataset to feed to the machine learning model, and this process of feeding the data to our Designed ML model is called Training. In the training stage, the model learns the behavior of data, capable of handling different forms of data to better suit the model, draws conclusion from the data and finally predicts the end results using the model.

This technique of training helps a user to know the output of the designed machine learning model for the given problem, the inputs given to the model, and the output that is obtained at the end of the model.

But as machine learning model engineers, we might doubt the applicability of the model for the problem and have questions like, is the developed Machine learning model best suited for the problem, how accurate the model is, how can we say this is the best model that suits the given problem statement and what are the measures that describe model performance?

In order to get clarity on the above questions, there is a technique called Model Evaluation, that describes the performance of the model and helps us understand if the designed model is suitable for the given problem statement or not.

This article helps you to know, the various measures involved in calculating performance of a model for a particular problem and other key aspects involved.

## What is Model Evaluation?

This technique of Evaluation helps us to know which algorithm best suits the given dataset for solving a particular problem. Likewise, in terms of Machine Learning it is called as “Best Fit”. It evaluates the performance of different Machine Learning models, based on the same input dataset. The method of evaluation focuses on accuracy of the model, in predicting the end outcomes.

Out of all the different algorithms we use in the stage, we choose the algorithm that gives more accuracy for the input data and is considered as the best model as it better predicts the outcome. The accuracy is considered as the main factor, when we work on solving different problems using machine learning. If the accuracy is high, the model predictions on the given data are also true to the maximum possible extent.

There are several stages in solving an ML problem like collection of dataset, defining the problem, brainstorming on the given data, preprocessing, transformation, training the model and evaluating. Even though there are several stages, the stage of Evaluation of a ML model is the most crucial stage, because it gives us an idea of the accuracy of model prediction. The performance and usage of the ML model is decided in terms of accuracy measures at the end.

## Model Evaluation Techniques

We have known that the model evaluation is an Integral part in Machine Learning. Initially, the dataset is divided into two types, they are “Training dataset and “Test dataset”. We build the machine learning model using the training dataset to see the functionality of the model. But we evaluate the designed Model using a test dataset, which consists of unseen or unknown samples of the data that are not used for training purposesEvaluation of a model tells us how accurate the results wereIf we use the training dataset for evaluation of the model, for any instance of the training data it will always show the correct predictions for the given problem with high accuracy measures, in that case our model is not adequately effective to use.

There are two methods that are used to evaluate a model performance. They are

1. Holdout
2. Cross Validation

The Holdout method is used to evaluate the model performance and uses two types of data for testing and training. The test data is used to calculate the performance of the model whereas it is trained using the training data set.  This method is used to check how well the machine learning model developed using different algorithm techniques performs on unseen samples of dataThis approach is simple, flexible and fast.

Cross-validation is a procedure of dividing the whole dataset into data samples, and then evaluating the machine learning model using the other samples of data to know accuracy of the model. i.e., we train the model using subset of data and we evaluate it with a complementary data subset. We can calculate cross validation based on the following 3 methods, namely

1. Validation
2. Leave one out cross validation (LOOCV)
3. K-Fold Cross Validation

In the method of validation, we split the given dataset into 50% of training and 50% for testing purpose. The main drawback in this method is that the remaining 50% of data that is subjected to testing may contain some crucial information that may be lost while training the model. So, this method doesn’t work properly due to high bias.

In the method of LOOCV, we train all the datasets in our model and leave a single data point for testing purpose. This method aims at exhibiting lower bias, but there are some chances that this method might fail because, the data-point that has been left out may be an outlier in the given data; and in that case we cannot produce better results with good accuracy.

K-fold cross validation is a popular method used for evaluation of a Machine Learning model. It works by splitting the data into k-parts. Each split of the data is called a fold. Here we train all the k subsets of data to the model, and then we leave out one (k-1) subset to perform evaluation on the trained model. This method results in high accuracy and produces data with less bias.

## Types of Predictive Models

Predictive models are used to predict the outcomes from the given data by using a developed ML model. Before getting the actual output from the model, we can predict the outcomes with the help of given data. The prediction models are widely used in machine learning, to guess the outcomes from the data before designing a model. There are different types of predictive models:

1. Classification model
2. Clustering model
3. Forecast model
4. Outlier model

A Classification model is used in decision making problems. It separates the given data into different categories, and this model is best suited to answer “Yes” or “No” questions. It is the simplest of all the predictive models.

Real Life Applications: Projects like Gender Classification, Fraud detection, Product Categorization, Malware classification, documents classification etc.

Clustering models are used to group the given data based on similar attributes. This model helps us to know how many groups are present in the given dataset and we can analyze what are the groups, which we should focus on to solve the given problem statement.

Real Life Applications: Projects like categorizing different people present in a classroom, types of customers in a bank, identifying fake news, spam filter, document analysis etc.

A forecast model learns from the historical data in order to predict the new data based on learning. It majorly deals with metric values.

Real Life Applications: Projects like weather forecast, sales forecast, stocks prices, Heart Rate Monitoring etc.

Outlier model focuses on identifying irrelevant data in the given dataset. If the data consists of outliers, we cannot get good results as the outliers have irrelevant data. The outliers may have categorical or numerical type of data associated with them.

Real Life Applications: Major applications are used in Retail Industries, Finance Industries, Quality Control, Fault Diagnosis, web analytics etc.

### Classification Metrics

In order to evaluate the performance of a Machine Learning model, there are some Metrics to know its performance and are applied for Regression and Classification algorithms. The different types of classification metrics are:

1. Classification Accuracy
2. Confusion Matrix
3. Logarithmic Loss
4. Area under Curve (AUC)
5. F-Measure

### Classification Accuracy

Classification accuracy is similar to the term Accuracy. It is the ratio of the correct predictions to the total number of Predictions made by the model from the given data.

We can get better accuracy if the given data samples have the same type of data related to the given problem statementIf the accuracy is high, the model is more accurate and we can use the model in the real world and for different types of applications also.

If the accuracy is less, it shows that the data samples are not correctly classified to suit the given problem.

### Confusion Matrix

It is a NxN matrix structure used for evaluating the performance of a classification model, where N is the number of classes that are predicted. It is operated on a test dataset in which the true values are known. The matrix lets us know about the number of incorrect and correct predictions made by a classifier and is used to find correctness of the model. It consists of values like True Positive, False Positive, True Negative, and False Negative, which helps in measuring Accuracy, Precision, Recall, Specificity, Sensitivity, and AUC curve. The above measures will talk about the model performance and compare with other models to describe how good it is.

There are 4 important terms in confusion matrix:

1. True Positives (TP): The cases in which our predictions are TRUE, and the actual output was also TRUE.
2. True Negatives (TN): The cases in which our predictions are FALSE, and the actual output was also FALSE.
3. False Positives (FP): The cases in which our predictions are TRUE, and the actual output was FALSE.
4. False Negative (FN): The cases in which our predictions are FALSE, and the actual output was TRUE.

The accuracy can be calculated by using the mean of True Positive and True Negative values of the total sample values. It tells us about the total number of predictions made by the model that were correct.

Precision is the ratio of Number of True Positives in the sample to the total Positive samples predicted by the classifier. It tells us about the positive samples that were correctly identified by the model.

Recall is the ratio of Number of True Positives in the sample to the sum of True Positive and False Negative samples in the data.

### F1 Score

It is also called as F-Measure. It is a best measure of the Test accuracy of the developed model. It makes our task easy by eliminating the need to calculate Precision and Recall separately to know about the model performance. F1 Score is the Harmonic mean of Recall and Precision. Higher the F1 Score, better the performance of the model. Without calculating Precision and Recall separately, we can calculate the model performance using F1 score as it is precise and robust.

Sensitivity is the ratio of Number of actual True Positive Samples to the sum of True Positive and False Positive Samples. It tells about the positive samples that are identified correctly with respect to all the positive data samples in the given data. It is also called as True Positive Rate.

Specificity is also called the True Negative Rate. It is the ratio of the Number of True Negatives in the sample to the sum of True negative and the False positive samples in the given dataset. It tells about the number of actual Negative samples that are correctly identified from the given dataset.

False positive rate is defined as 1-specificity. It is the ratio of number of False Positives in the sample to the sum of False positive and True Negative samples. It tells us about the Negative data samples that are classified as Positive, with respect to all Negative data samples.

For each value of sensitivity, we get a different value of specificity and they are associated as follows:

## Area Under the ROC Curve (AUC - ROC)

It is a widely used Evaluation Metric, mainly used for Binary ClassificationThe False positive rates and the True positive rates have the values ranging from 0 to 1The TPR and FPR are calculated with different threshold values and a graph is drawn to better understand about the data. Thus, the Area Under Curve is the plot between false positive rate and True positive rate at different values of [0,1].

### Logarithmic Loss

It is also called Log LossAs we know, the AUC ROC determines the model performance using the predicted probabilities, but it does not consider model capability to predict the higher probability of samples to be more likely positive. This technique is mostly used in Multi-class Classification. It is calculated to the negative average of the log of correctly predicted probabilities for each instance.

where,

• y_ij, indicates whether sample i belongs to class j or not
• p_ij, indicates the probability of sample i belonging to class j

### Regression Metrics

It helps to predict the state of outcome at any time with the help of independent variables that are correlated. There are mainly 3 different types of metrics used in regression. These metrics are designed in order to predict if the data is underfitted or overfitted for the better usage of the model.

They are:-

1. Mean Absolute Error (MAE)
2. Mean Squared Error (MSE)
3. Root Mean Squared Error (RMSE)

Mean Absolute Error is the average of the difference of the original values and the predicted values. It gives us an idea of how far the predictions are from the actual output. It doesn’t give clarity on whether the data is under fitted or over fitted. It is calculated as follows:

• The mean squared error is similar to the mean absolute error. It is computed by taking the average of the square of the difference between original and predicted values. With the help of squaring, large errors can be converted to small errors and large errors can be dealt with It is computed as follows.
• The root mean squared error is the root of the mean of the square of difference of the predicted and actual values of the given data. It is the most popular metric evolution technique used in regression problems. It follows a normal distribution and is based on the assumption that errors are unbiased. It is computed using the below formulae.

### Bias vs Variance

Bias is the difference between the Expected value and the Predicted value by our model. It is simply some assumptions made by the model to make the target function easier to learn. The low bias indicates fewer assumptions, whereas the high bias talks about more assumptions in the target data. It leads to underfitting of the model.

Variance takes all types of data including noise into it. The model considers the variance as something to learn, and the model learns too much from the trained data, and at the end the model fails in giving out accurate results to the given problem statement. In case of high variance, the model learns too much and it can lead to overfitting of the model.

Conclusion

While building a machine learning model for a given problem statement there are two important stages, namely training and testing. In the training stage, the models learn from the data and predict the outcomes at the end. But it is crucial that predictions made by the developed model are accurateThis is why the stage of testing is the most crucial stage, because it can guarantee how accurate the results were to implement for the given problem.

In this blog, we have discussed about various types of Evaluation techniques to achieve a good model that best suits a given problem statement with highly accurate results. We need to check all the above-mentioned parameters to be able to compare our model performance as compared to other models.

### Harsha Vardhan Garlapati

Blog Writer at KnowledgeHut

Harsha Vardhan Garlapati is a Data Science Enthusiast and loves working with data to draw meaningful insights from it and further convert those results and implement them in business growth. He is a final year undergraduate student and passionate about Data Science. He is a smart worker, passionate learner,  an Ice-Breaker and loves to participate in Hackathons to work on real time projects. He is a Toastmaster Member at S.R.K.R Toastmasters Club, a Public Speaker, a good Innovator and problem solver.

## What is Linear Regression in Machine Learning

Machine Learning, being a subset of Artificial Intelligence (AI), has been playing a dominant role in our daily lives. Data science engineers and developers working in various domains are widely using machine learning algorithms to make their tasks simpler and life easier. For example, certain machine learning algorithms enable Google Maps to find the fastest route to our destinations, allow Tesla to make driverless cars, help Amazon to generate almost 35% of their annual income, AccuWeather to get the weather forecast of 3.5 million locations weeks in advance, Facebook to automatically detect faces and suggest tags and so on.In statistics and machine learning, linear regression is one of the most popular and well understood algorithms. Most data science enthusiasts and machine learning  fanatics begin their journey with linear regression algorithms. In this article, we will look into how linear regression algorithm works and how it can be efficiently used in your machine learning projects to build better models.Linear Regression is one of the machine learning algorithms where the result is predicted by the use of known parameters which are correlated with the output. It is used to predict values within a continuous range rather than trying to classify them into categories. The known parameters are used to make a continuous and constant slope which is used to predict the unknown or the result.What is a Regression Problem?Majority of the machine learning algorithms fall under the supervised learning category. It is the process where an algorithm is used to predict a result based on the previously entered values and the results generated from them. Suppose we have an input variable ‘x’ and an output variable ‘y’ where y is a function of x (y=f{x}). Supervised learning reads the value of entered variable ‘x’ and the resulting variable ‘y’ so that it can use those results to later predict a highly accurate output data of ‘y’ from the entered value of ‘x’. A regression problem is when the resulting variable contains a real or a continuous value. It tries to draw the line of best fit from the data gathered from a number of points.For example, which of these is a regression problem?How much gas will I spend if I drive for 100 miles?What is the nationality of a person?What is the age of a person?Which is the closest planet to the Sun?Predicting the amount of gas to be spent and the age of a person are regression problems. Predicting nationality is categorical and the closest planet to the Sun is discrete.What is Linear Regression?Let’s say we have a dataset which contains information about the relationship between ‘number of hours studied’ and ‘marks obtained’. A number of students have been observed and their hours of study along with their grades are recorded. This will be our training data. Our goal is to design a model that can predict the marks if number of hours studied is provided. Using the training data, a regression line is obtained which will give minimum error. This linear equation is then used to apply for a new data. That is, if we give the number of hours studied by a student as an input, our model should be able to predict their mark with minimum error.Hypothesis of Linear RegressionThe linear regression model can be represented by the following equation:where,Y is the predicted valueθ₀ is the bias term.θ₁,…,θn are the model parametersx₁, x₂,…,xn are the feature values.The above hypothesis can also be represented byWhere, θ is the model’s parameter vector including the bias term θ₀; x is the feature vector with x₀ =1Y (pred) = b0 + b1*xThe values b0 and b1 must be chosen so that the error is minimum. If sum of squared error is taken as a metric to evaluate the model, then the goal is to obtain a line that best reduces the error.If we don’t square the error, then the positive and negative points will cancel each other out.For a model with one predictor,Exploring ‘b1’If b1 > 0, then x (predictor) and y(target) have a positive relationship. That is an increase in x will increase y.If b1 < 0, then x (predictor) and y(target) have a negative relationship. That is an increase in x will decrease y.Exploring ‘b0’If the model does not include x=0, then the prediction will become meaningless with only b0. For example, we have a dataset that relates height(x) and weight(y). Taking x=0 (that is height as 0), will make the equation have only b0 value which is completely meaningless as in real-time height and weight can never be zero. This resulted due to considering the model values beyond its scope.If the model includes value 0, then ‘b0’ will be the average of all predicted values when x=0. But, setting zero for all the predictor variables is often impossible.The value of b0 guarantees that the residual will have mean zero. If there is no ‘b0’ term, then the regression will be forced to pass over the origin. Both the regression coefficient and prediction will be biased.How does Linear Regression work?Let’s look at a scenario where linear regression might be useful: losing weight. Let us consider that there’s a connection between how many calories you take in and how much you weigh; regression analysis can help you understand that connection. Regression analysis will provide you with a relation which can be visualized into a graph in order to make predictions about your data. For example, if you’ve been putting on weight over the last few years, it can predict how much you’ll weigh in the next ten years if you continue to consume the same amount of calories and burn them at the same rate.The goal of regression analysis is to create a trend line based on the data you have gathered. This then allows you to determine whether other factors apart from the amount of calories consumed affect your weight, such as the number of hours you sleep, work pressure, level of stress, type of exercises you do etc. Before taking into account, we need to look at these factors and attributes and determine whether there is a correlation between them. Linear Regression can then be used to draw a trend line which can then be used to confirm or deny the relationship between attributes. If the test is done over a long time duration, extensive data can be collected and the result can be evaluated more accurately. By the end of this article we will build a model which looks like the below picture i.e, determine a line which best fits the data.How do we determine the best fit line?The best fit line is considered to be the line for which the error between the predicted values and the observed values is minimum. It is also called the regression line and the errors are also known as residuals. The figure shown below shows the residuals. It can be visualized by the vertical lines from the observed data value to the regression line.When to use Linear Regression?Linear Regression’s power lies in its simplicity, which means that it can be used to solve problems across various fields. At first, the data collected from the observations need to be collected and plotted along a line. If the difference between the predicted value and the result is almost the same, we can use linear regression for the problem.Assumptions in linear regressionIf you are planning to use linear regression for your problem then there are some assumptions you need to consider:The relation between the dependent and independent variables should be almost linear.The data is homoscedastic, meaning the variance between the results should not be too much.The results obtained from an observation should not be influenced by the results obtained from the previous observation.The residuals should be normally distributed. This assumption means that the probability density function of the residual values is normally distributed at each independent value.You can determine whether your data meets these conditions by plotting it and then doing a bit of digging into its structure.Few properties of Regression LineHere are a few features a regression line has:Regression passes through the mean of independent variable (x) as well as mean of the dependent variable (y).Regression line minimizes the sum of “Square of Residuals”. That’s why the method of Linear Regression is known as “Ordinary Least Square (OLS)”. We will discuss more in detail about Ordinary Least Square later on.B1 explains the change in Y with a change in x  by one unit. In other words, if we increase the value of ‘x’ it will result in a change in value of Y.Finding a Linear Regression lineLet’s say we want to predict ‘y’ from ‘x’ given in the following table and assume they are correlated as “y=B0+B1∗x”xyPredicted 'y'12Β0+B1∗121Β0+B1∗233Β0+B1∗346Β0+B1∗459Β0+B1∗5611Β0+B1∗6713Β0+B1∗7815Β0+B1∗8917Β0+B1∗91020Β0+B1∗10where,Std. Dev. of x3.02765Std. Dev. of y6.617317Mean of x5.5Mean of y9.7Correlation between x & y0.989938If the Residual Sum of Square (RSS) is differentiated with respect to B0 & B1 and the results equated to zero, we get the following equation:B1 = Correlation * (Std. Dev. of y/ Std. Dev. of x)B0 = Mean(Y) – B1 * Mean(X)Putting values from table 1 into the above equations,B1 = 2.64B0 = -2.2Hence, the least regression equation will become –Y = -2.2 + 2.64*xxY - ActualY - Predicted120.44213.08335.72468.36591161113.6471316.2881518.9291721.56102024.2As there are only 10 data points, the results are not too accurate but if we see the correlation between the predicted and actual line, it has turned out to be very high; both the lines are moving almost together and here is the graph for visualizing our predicted values:Model PerformanceAfter the model is built, if we see that the difference in the values of the predicted and actual data is not much, it is considered to be a good model and can be used to make future predictions. The amount that we consider “not much” entirely depends on the task you want to perform and to what percentage the variation in data can be handled. Here are a few metric tools we can use to calculate error in the model-R – Square (R2)Total Sum of Squares (TSS): total sum of squares (TSS) is a quantity that appears as part of a standard way of presenting results of such an analysis. Sum of squares is a measure of how a data set varies around a central number (like the mean). The Total Sum of Squares tells how much variation there is in the dependent variable.TSS = Σ (Y – Mean[Y])2Residual Sum of Squares (RSS): The residual sum of squares tells you how much of the dependent variable’s variation your model did not explain. It is the sum of the squared differences between the actual Y and the predicted Y.RSS = Σ (Y – f[Y])2(TSS – RSS) measures the amount of variability in the response that is explained by performing the regression.Properties of R2R2 always ranges between 0 to 1.R2 of 0 means that there is no correlation between the dependent and the independent variable.R2 of 1 means the dependent variable can be predicted from the independent variable without any error. An R2 between 0 and 1 indicates the extent to which the dependent variable is predictable. An R2 of 0.20 means that there is 20% of the variance in Y is predictable from X; an R2 of 0.40 means that 40% is predictable; and so on.Root Mean Square Error (RMSE)Root Mean Square Error (RMSE) is the standard deviation of the residuals (prediction errors). The formula for calculating RMSE is:Where N : Total number of observationsWhen standardized observations are used as RMSE inputs, there is a direct relationship with the correlation coefficient. For example, if the correlation coefficient is 1, the RMSE will be 0, because all of the points lie on the regression line (and therefore there are no errors).Mean Absolute Percentage Error (MAPE)There are certain limitations to the use of RMSE, so analysts prefer MAPE over RMSE which gives error in terms of percentages so that different models can be considered for the task and see how they perform. Formula for calculating MAPE can be written as:Where N : Total number of observationsFeature SelectionFeature selection is the automatic selection of attributes for your data that are most relevant to the predictive model you are working on. It seeks to reduce the number of attributes in the dataset by eliminating the features which are not required for the model construction. Feature selection does not totally eliminate an attribute which is considered for the model, rather it mutes that particular characteristic and works with the features which affects the model.Feature selection method aids your mission to create an accurate predictive model. It helps you by choosing features that will give you as good or better accuracy whilst requiring less data. Feature selection methods can be used to identify and remove unnecessary, irrelevant and redundant attributes from the data that do not contribute to the accuracy of the model or may even decrease the accuracy of the model. Having fewer attributes is desirable because it reduces the complexity of the model, and a simpler model is easier to understand, explain and to work with.Feature Selection Algorithms:Filter Method: This method involves assigning scores to individual features and ranking them. The features that have very little to almost no impact are removed from consideration while constructing the model.Wrapper Method: Wrapper method is quite similar to Filter method except the fact that it considers attributes in a group i.e. a number of attributes are taken and checked whether they are having an impact on the model and if not another combination is applied.Embedded Method: Embedded method is the best and most accurate of all the algorithms. It learns the features that affect the model while the model is being constructed and takes into consideration only those features. The most common type of embedded feature selection methods are regularization methods.Cost FunctionCost function helps to figure out the best possible plots which can be used to draw the line of best fit for the data points. As we want to reduce the error of the resulting value we change the process of finding out the actual result to a process which can reduce the error between the predicted value and the actual value.Here, J is the cost function.The above function is made in this format to calculate the error difference between the predicted values and the plotted values. We take the square of the summation of all the data points and divide it by the total number of data points. This cost function J is also called the Mean Squared Error (MSE) function. Using this MSE function we are going to predict values such that the MSE value settles at the minima, reducing the cost function.Gradient DescentGradient Descent is an optimization algorithm that helps machine learning models to find out paths to a minimum value using repeated steps. Gradient descent is used to minimize a function so that it gives the lowest output of that function. This function is called the Loss Function. The loss function shows us how much error is produced by the machine learning model compared to actual results. Our aim should be to lower the cost function as much as possible. One way of achieving a low cost function is by the process of gradient descent. Complexity of some equations makes it difficult to use, partial derivative of the cost function with respect to the considered parameter can provide optimal coefficient value. You may refer to the article on Gradient Descent for Machine Learning.Simple Linear RegressionOptimization is a big part of machine learning and almost every machine learning algorithm has an optimization technique at its core for increased efficiency. Gradient Descent is such an optimization algorithm used to find values of coefficients of a function that minimizes the cost function. Gradient Descent is best applied when the solution cannot be obtained by analytical methods (linear algebra) and must be obtained by an optimization technique.Residual Analysis: Simple linear regression models the relationship between the magnitude of one variable and that of a second—for example, as x increases, y also increases. Or as x increases, y decreases. Correlation is another way to measure how two variables are related. The models done by simple linear regression estimate or try to predict the actual result but most often they deviate from the actual result. Residual analysis is used to calculate by how much the estimated value has deviated from the actual result.Null Hypothesis and p-value: During feature selection, null hypothesis is used to find which attributes will not affect the result of the model. Hypothesis tests are used to test the validity of a claim that is made about a particular attribute of the model. This claim that’s on trial, in essence, is called the null hypothesis. A p-value helps to determine the significance of the results. p-value is a number between 0 and 1 and is interpreted in the following way:A small p-value (less than 0.05) indicates a strong evidence against the null hypothesis, so the null hypothesis is to be rejected.A large p-value (greater than 0.05) indicates weak evidence against the null hypothesis, so the null hypothesis is to be considered.p-value very close to the cut-off (equal to 0.05) is considered to be marginal (could go either way). In this case, the p-value should be provided to the readers so that they can draw their own conclusions.Ordinary Least SquareOrdinary Least Squares (OLS), also known as Ordinary least squares regression or least squared errors regression is a type of linear least squares method for estimating the unknown parameters in a linear regression model. OLS chooses the parameters for a linear function, the goal of which is to minimize the sum of the squares of the difference of the observed variables and the dependent variables i.e. it tries to attain a relationship between them. There are two types of relationships that may occur: linear and curvilinear. A linear relationship is a straight line that is drawn through the central tendency of the points; whereas a curvilinear relationship is a curved line. Association between the variables are depicted by using a scatter plot. The relationship could be positive or negative, and result variation also differs in strength.The advantage of using Ordinary Least Squares regression is that it can be easily interpreted and is highly compatible with recent computers’ built-in algorithms from linear algebra. It can be used to apply to problems with lots of independent variables which can efficiently conveyed to thousands of data points. In Linear Regression, OLS is used to estimate the unknown parameters by creating a model which will minimize the sum of the squared errors between the observed data and the predicted one.Let us simulate some data and look at how the predicted values (Yₑ) differ from the actual value (Y):import pandas as pd import numpy as np from matplotlib import pyplot as plt # Generate 'random' data np.random.seed(0) X = 2.5 * np.random.randn(100) + 1.5   # Array of 100 values with mean = 1.5, stddev = 2.5 res = 0.5 * np.random.randn(100)         # Generate 100 residual terms y = 2 + 0.3 * X + res                   # Actual values of Y # Create pandas dataframe to store our X and y values df = pd.DataFrame(     {'X': X,       'y': y} ) # Show the first five rows of our dataframe df.head()XY05.9101314.71461512.5003932.07623823.9468452.54881137.1022334.61536846.1688953.264107To estimate y using the OLS method, we need to calculate xmean and ymean, the covariance of X and y (xycov), and the variance of X (xvar) before we can determine the values for alpha and beta.# Calculate the mean of X and y xmean = np.mean(X) ymean = np.mean(y) # Calculate the terms needed for the numator and denominator of beta df['xycov'] = (df['X'] - xmean) * (df['y'] - ymean) df['xvar'] = (df['X'] - xmean)**2 # Calculate beta and alpha beta = df['xycov'].sum() / df['xvar'].sum() alpha = ymean - (beta * xmean) print(f'alpha = {alpha}') print(f'beta = {beta}')alpha = 2.0031670124623426 beta = 0.3229396867092763Now that we have an estimate for alpha and beta, we can write our model as Yₑ = 2.003 + 0.323 X, and make predictions:ypred = alpha + beta * XLet’s plot our prediction ypred against the actual values of y, to get a better visual understanding of our model.# Plot regression against actual data plt.figure(figsize=(12, 6)) plt.plot(X, ypred) # regression line plt.plot(X, y, 'ro')   # scatter plot showing actual data plt.title('Actual vs Predicted') plt.xlabel('X') plt.ylabel('y') plt.show()The blue line in the above graph is our line of best fit, Yₑ = 2.003 + 0.323 X.  If you observe the graph carefully, you will notice that there is a linear relationship between X and Y. Using this model, we can predict Y from any values of X. For example, for X = 8,Yₑ = 2.003 + 0.323 (8) = 4.587RegularizationRegularization is a type of regression that is used to decrease the coefficient estimates down to zero. This helps to eliminate the data points that don’t actually represent the true properties of the model, but have appeared by random chance. The process is done by identifying the points which have deviated from the line of best-fit by a large extent. Earlier we saw that to estimate the regression coefficients β in the least squares method, we must minimize the term Residual Sum of Squares (RSS). Let the RSS equation in this case be:The general linear regression model can be expressed using a condensed formula:Here, β=[β0 ,β1, ….. βp]The RSS value will adjust the coefficient, β based on the training data. If the resulting data deviates too much from the training data, then the estimated coefficients won’t generalize well to the future data. This is where regularization comes in and shrinks or regularizes these learned estimates towards zero.Ridge regressionRidge regression is very similar to least squares, except that the Ridge coefficients are estimated by minimizing a different quantity. In particular, the Ridge regression coefficients β are the values that minimize the following quantity:Here, λ is the tuning parameter that decides how much we want to penalize the flexibility of the model. λ controls the relative impact of the two components: RSS and the penalty term. If λ = 0, the Ridge regression will produce a result similar to least squares method. If λ → ∞, all estimated coefficients tend to zero. Ridge regression produces different estimates for different values of λ. The optimal choice of λ is crucial and should be done with cross-validation. The coefficient estimates produced by ridge regression method is also known as the L2 norm.The coefficients generated by Ordinary Least Squares method is independent of scale, which means that if each input variable is multiplied by a constant, the corresponding coefficient will be divided by the same constant, as a result of which the multiplication of the coefficient and the input variables will remain the same. The same is not true for ridge regression and we need to bring the coefficients to the same scale before we perform the process. To standardize the variables, we must subtract their means and divide it by their standard deviations.Lasso RegressionLeast Absolute Shrinkage and Selection Operator (LASSO) regression also shrinks the coefficients by adding a penalty to the sum of squares of the residuals, but the lasso penalty has a slightly different effect. The lasso penalty is the sum of the absolute values of the coefficient vector, which corresponds to its L1 norm. Hence, the lasso estimate is defined by:Similar to ridge regression, the input variables need to be standardized. The lasso penalty makes the solution nonlinear, and there is no closed-form expression for the coefficients as in ridge regression. Instead, the lasso solution is a quadratic programming problem and there are available efficient algorithms that compute the entire path of coefficients that result for different values of λ with the same computational cost as for ridge regression.The lasso penalty had the effect of gradually reducing some coefficients to zero as the regularization increases. For this reason, the lasso can be used for the continuous selection of a subset of features.Linear Regression with multiple variablesLinear regression with multiple variables is also known as "multivariate linear regression". We now introduce notation for equations where we can have any number of input variables.x(i)j=value of feature j in the ith training examplex(i)=the input (features) of the ith training examplem=the number of training examplesn=the number of featuresThe multivariable form of the hypothesis function accommodating these multiple features is as follows:hθ(x)=θ0+θ1x1+θ2x2+θ3x3+⋯+θnxnIn order to develop intuition about this function, we can think about θ0 as the basic price of a house, θ1 as the price per square meter, θ2 as the price per floor, etc. x1 will be the number of square meters in the house, x2 the number of floors, etc.Using the definition of matrix multiplication, our multivariable hypothesis function can be concisely represented as:This is a vectorization of our hypothesis function for one training example; see the lessons on vectorization to learn more.Remark: Note that for convenience reasons in this course we assume x0 (i) =1 for (i∈1,…,m). This allows us to do matrix operations with θ and x. Hence making the two vectors ‘θ’and x(i) match each other element-wise (that is, have the same number of elements: n+1).Multiple Linear RegressionHow is it different?In simple linear regression we use a single independent variable to predict the value of a dependent variable whereas in multiple linear regression two or more independent variables are used to predict the value of a dependent variable. The difference between the two is the number of independent variables. In both cases there is only a single dependent variable.MulticollinearityMulticollinearity tells us the strength of the relationship between independent variables. Multicollinearity is a state of very high intercorrelations or inter-associations among the independent variables. It is therefore a type of disturbance in the data, and if present in the data the statistical inferences made about the data may not be reliable. VIF (Variance Inflation Factor) is used to identify the Multicollinearity. If VIF value is greater than 4, we exclude that variable from our model.There are certain reasons why multicollinearity occurs:It is caused by an inaccurate use of dummy variables.It is caused by the inclusion of a variable which is computed from other variables in the data set.Multicollinearity can also result from the repetition of the same kind of variable.Generally occurs when the variables are highly correlated to each other.Multicollinearity can result in several problems. These problems are as follows:The partial regression coefficient due to multicollinearity may not be estimated precisely. The standard errors are likely to be high.Multicollinearity results in a change in the signs as well as in the magnitudes of the partial regression coefficients from one sample to another sample.Multicollinearity makes it tedious to assess the relative importance of the independent variables in explaining the variation caused by the dependent variable.Iterative ModelsModels should be tested and upgraded again and again for better performance. Multiple iterations allows the model to learn from its previous result and take that into consideration while performing the task again.Making predictions with Linear RegressionLinear Regression can be used to predict the value of an unknown variable using a known variable by the help of a straight line (also called the regression line). The prediction can only be made if it is found that there is a significant correlation between the known and the unknown variable through both a correlation coefficient and a scatterplot.The general procedure for using regression to make good predictions is the following:Research the subject-area so that the model can be built based on the results produced by similar models. This research helps with the subsequent steps.Collect data for appropriate variables which have some correlation with the model.Specify and assess the regression model.Run repeated tests so that the model has more data to work with.To test if the model is good enough observe whether:The scatter plot forms a linear pattern.The correlation coefficient r, has a value above 0.5 or below -0.5. A positive value indicates a positive relationship and a negative value represents a negative relationship.If the correlation coefficient shows a strong relationship between variables but the scatter plot is not linear, the results can be misleading. Examples on how to use linear regression have been shown earlier.Data preparation for Linear RegressionStep 1: Linear AssumptionThe first step for data preparation is checking for the variables which have some sort of linear correlation between the dependent and the independent variables.Step 2: Remove NoiseIt is the process of reducing the number of attributes in the dataset by eliminating the features which have very little to no requirement for the construction of the model.Step 3: Remove CollinearityCollinearity tells us the strength of the relationship between independent variables. If two or more variables are highly collinear, it would not make sense to keep both the variables while evaluating the model and hence we can keep one of them.Step 4: Gaussian DistributionsThe linear regression model will produce more reliable results if the input and output variables have a Gaussian distribution. The Gaussian theorem states that  states that a sample mean from an infinite population is approximately normal, or Gaussian, with mean the same as the underlying population, and variance equal to the population variance divided by the sample size. The approximation improves as the sample size gets large.Step 5: Rescale InputsLinear regression model will produce more reliable predictions if the input variables are rescaled using standardization or normalization.Linear Regression with statsmodelsWe have already discussed OLS method, now we will move on and see how to use the OLS method in the statsmodels library. For this we will be using the popular advertising dataset. Here, we will only be looking at the TV variable and explore whether spending on TV advertising can predict the number of sales for the product. Let’s start by importing this csv file as a pandas dataframe using read_csv():# Import and display first five rows of advertising dataset advert = pd.read_csv('advertising.csv') advert.head()TVRadioNewspaperSales0230.137.869.222.1144.539.345.110.4217.245.969.312.03151.541.358.516.54180.810.858.417.9Now we will use statsmodels’ OLS function to initialize simple linear regression model. It will take the formula y ~ X, where X is the predictor variable (TV advertising costs) and y is the output variable (Sales). Then, we will fit the model by calling the OLS object’s fit() method.import statsmodels.formula.api as smf # Initialise and fit linear regression model using statsmodels model = smf.ols('Sales ~ TV', data=advert) model = model.fit()Once we have fit the simple regression model, we can predict the values of sales based on the equation we just derived using the .predict method and also visualise our regression model by plotting sales_pred against the TV advertising costs to find the line of best fit.# Predict values sales_pred = model.predict() # Plot regression against actual data plt.figure(figsize=(12, 6)) plt.plot(advert['TV'], advert['Sales'], 'o')       # scatter plot showing actual data plt.plot(advert['TV'], sales_pred, 'r', linewidth=2)   # regression line plt.xlabel('TV Advertising Costs') plt.ylabel('Sales') plt.title('TV vs Sales') plt.show()In the above graph, if you notice you will see that there is a positive linear relationship between TV advertising costs and Sales. You may also summarize by saying that spending more on TV advertising predicts a higher number of sales.Linear Regression with scikit-learnLet us learn to implement linear regression models using sklearn. For this model as well, we will continue to use the advertising dataset but now we will use two predictor variables to create a multiple linear regression model. Yₑ = α + β₁X₁ + β₂X₂ + … + βₚXₚ, where p is the number of predictors.In our example, we will be predicting Sales using the variables TV and Radio i.e. our model can be written as:Sales = α + β₁*TV + β₂*Radiofrom sklearn.linear_model import LinearRegression # Build linear regression model using TV and Radio as predictors # Split data into predictors X and output Y predictors = ['TV', 'Radio'] X = advert[predictors] y = advert['Sales'] # Initialise and fit model lm = LinearRegression() model = lm.fit(X, y) print(f'alpha = {model.intercept_}') print(f'betas = {model.coef_}')alpha = 4.630879464097768 betas = [0.05444896 0.10717457]model.predict(X)Now that we have fit a multiple linear regression model to our data, we can predict sales from any combination of TV and Radio advertising costs. For example, you want to know how many sales we would make if we invested $600 in TV advertising and$300 in Radio advertising. You can simply find it out by:new_X = [[600, 300]] print(model.predict(new_X))[69.4526273]We get the output as 69.45 which means if we invest $600 on TV and$300 on Radio advertising, we can expect to sell 69 units approximately.SummaryLet us sum up what we have covered in this article so far —How to understand a regression problemWhat is linear regression and how it worksOrdinary Least Square method and RegularizationImplementing Linear Regression in Python using statsmodel and sklearn libraryWe have discussed about a couple of ways to implement linear regression and build efficient models for certain business problems. If you are inspired by the opportunities provided by machine learning, enrol in our  Data Science and Machine Learning Courses for more lucrative career options in this landscape.
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What is Linear Regression in Machine Learning

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## Combining Models – Python Machine Learning

This article will help you in the installation of Python 3  on macOS. You will learn the basics of configuring the environment to get started with Python.Brief introduction to PythonPython is an Interpreted programming language that is very popular these days due to its easy learning curve and simple syntax. Python finds use in many applications and for programming the backend code of websites. It is also very popular for data analysis across industries ranging from medical/scientific research purposes to retail, finances, entertainment, media and so on.When writing a python program or program in any other language, people usually use something called an IDE or Integrated Development Environment that includes everything you need to write a program. It has an inbuilt text editor to write the program and a debugger to debug the programs as well. PyCharm is a well-known IDE for writing python programs.Latest version of pythonThe latest version of python is python3 and the latest release is python3.9.0.Installation linksFor downloading python and the documentation for MacOS, visit the official website and go to the downloads section, from where you can download the latest python version for MacOS.Key terms (pip, virtual environment, path etc.)pip:pip is a package manager to simplify the installation of python packages. To install pip, run the below command on the terminal:curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py.If you install python by using brew which is a package management to simplify installation of software on macOs, it installs other dependent  packages as well along with python3  like pip etc.virtual environment:The purpose of virtual environments is to have a separate space where you can install packages which are specific to a certain project. For example if you have a lot of flask or Django-based applications and not all the applications are using the same version, we use virtual env wherein each project will have its own version.In order to use a virtual environment you need to be on the python 3.x version. Let’s understand how to create the virtual environment. You do not need any library as it comes along with standard python installation.So to create a new virtual env, run the below command:python3 -m venv demo-m expects a module name which is venv in this case, so with this python searches your sys.path and executes the module as the main module.Venv expects an environment name that you must create.Now you should have a new environment called demo. Let’s activate this virtual env by running the below command:source demo/bin/activateAfter running this, the environment is activated and you can see the environment name in the terminal. Another way to check if the env is activated is by running which python. You will see the python that you are using with this project env, and the version that it will use is the same that you used to create the environment.Getting and Installing MacPython:For MacOS, python usually comes pre-installed, so to check if python is installed open the terminal in the mac and use python --version to confirm the same. You can also see what is the default python version installed, which is usually python2.x by default. However, Python2.x is going to get deprecated soon, and with everyone moving to python3.x ,we will go with the latest python3 installation.Installation stepsFor downloading python, visit the official website and go to the downloads section. You can download the latest python version for MacOS as shown below:It will download a pkg file. Click on that file to start the installation wizard. You can continue with default settings. If you want to change the install location, however,  you can change it, and then continue and finish the installation with the rest of the default settings.Once the installation is finished it would have created the python 3.x directory in the application folder. Just open the application folder and verify the same.Now you have python3.x installed.To verify it from the terminal, go to the terminal and check the version of python by using python --version command. So you will still see it is showing the old default python version, Now instead if you use python3 explicitly like `python3 –version, you can see the version that you have installed with python3 version.Once the installation is finished it would have created a python3.x directory in the application folder. Open the application folder and verify the same.You can also install python3 on mac by using brew which is a package management to simplify installation of software on macOs.brew install python3brew will install other dependent  packages as well along with python3  like pip etcSetting pathSuppose you have installed a new python 3  version but when you type python it still shows the default python2 version which comes by default in mac os. To solve this, add an alias by runningalias python=python3Add this line in the file called .bash_profile present in home directory. In case this file is not present, you can create it, save the changes and restart the terminal by closing it. Next, open the terminal and run python and hit enter. You should see the latest python3 that you have installed.Sometimes when you type python or python3 explicitly, it does not work even if you have installed the python. You get the message, “command is not found”. This means the command is not present in the directories used by the machine for lookup. Let’s  check the directories where the machine is looking for commands by runningecho $PATHIt will list all your directories where the machine looks for commands. This will vary from machine to machine. If the command that you are trying is not under the directory path listed by echo, that command will not work. It will throw an error saying command is not present, until you provide the full path of the directory where it's installed.Now let’s open the file .bash_profile and add the directory path where python is installed to the current path env variableFor example let’s add the following lines in that bash_profile file which will add the below directory to the current env variable. This can vary from machine to machine based on the installed location.PATH=”/Library/Frameworks/Python.framework/Versions/3.7/bin:${PATH}”export PATHSave the changes and restart the terminal. Open the terminal now and run echo \$PATH again and see the above path that you added for python3. When you now type python3 command, you should see it working.  Also, if you are trying to import a package that you have installed and it says that it cannot find that package, this means pip install is installing the packages in the different version of python directory. Make sure the location of the package is in the site-packages directory of the version of the python that you are using. You can see the location of the package that you are trying to import by running  pip show The above command will have a location field in which you can see and cross verify the path.9. How to run python codeTo run python code just run the commandpython Installing Additional Python Packages:If you want to see what all packages are installed in the env, run the command pip3 list which will list down the current packages installed. Let’s say you want to install request library. You can just install it by running pip3 install requests. Now try running pip3 list again, to see this requests lib installed in this env.Directory as package for distribution:Inside the python project or directory you should have a file called __init__.py. You can create this file by a simple touch command, and this file does not need to have any data inside it, All it has to do is to exist inside the directory, for that to work as a package.Documentation links for pythonPython DocConclusionThis article will help you with stepwise instructions on the installation of python on mac.