Leads, lags and float are concepts used in schedule development process. The process of schedule development includes: identification of all activities, sequencing all activities based on dependency, estimating duration of each activity and finalizing the schedule.
Leads, lags and float are used as part of activity sequencing process. All activities once sequenced will form a schedule network diagram. Let us first look at the definition of all these three attributes.
Float (also known as slack) is the amount of time by which the start of an activity can be delayed without delaying the project completion time. Every task will have following set of start and finish time.
Float time of an activity can be calculated by taking the difference between Late Start (LS) and Early Start (ES) OR between Late Finish (LF) and Early Finish (EF).
Float = LS-ES OR
A positive float time indicates the flexibility we will have in delaying the specific activity without delaying the project completion time.
Typically, while doing scheduling, the critical path tasks will have zero float and the non-critical path tasks will have a positive float. That means non-critical path tasks can be delayed to certain extent without compromising on the project completion time. Float time information of tasks is very useful to the project team for taking scheduling decisions when there will be resource constraints.
Lag is the amount of wait time between two tasks. Or in other words, lag is the amount of time by which a successor activity will be delayed. Lag can be used in all the four logical relationships in scheduling, such as Finish-to-start (FS), start-to-start (SS), finish-to-finish (FS) and start-to-finish (SF).
In below example, Task A and B have a Finish to Start (FS) relationship. Ideally both A and B should get finished on the 12th day. But when we insert a waiting of time of 2 days before B can start, then both A and B will get completed only on the 14th day.
Lead is the amount of time a successor task can be accelerated. Lead can applied only on finish-to-start relationship between two activities. We can see the below example. In the below example, task B can start 2 days before the completion of task A. Hence the start of task B, which ideally would have been on 6th day, will not start on 4th day.
Float, lead and lag are very important concepts and information for the scheduling team. These are used to optimally identify the dependencies and the associated constraints. Float information is useful in resource allocation when there are resource constraints. Lead is used for accelerating start of tasks (fast tracking) for reducing project timelines. Lag is used for ensuring that required idle or wait time after a task is appropriately provisioned.
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