Lead VS Lag
Every Project manager know the importance of leads and lags. PM’s work with both of them very frequently, if not daily. Leads and lags are very important in creating a Project Schedule Network Diagram. A Project Manager should have a good understanding of leads and lags in order to schedule the project optimally and execute the project effectively.
There are different types of dependencies that can be present between two activities. We see Leads and lags between two activities when they have a Finish-to-Start (FS) dependency between them.
In the above figures, Activity A is the Predecessor activity, and Activity B is the Successor Activity.
Following figure shows exactly how this looks in a Project Management Application (MS Project).
Now let us see, what Lead and Lag are in project management..
What is Lead time?
Lead time is the amount of time a successor activity can be advanced with respect to a predecessor activity.
What is Lag time?
Lag time is the amount of time a successor activity can be delayed with respect to a predecessor activity.
Lead and Lag Example:
Suppose in the above, Activity A takes 10 days to complete, and Activity B takes 5 days to complete, and both activities has a FS dependency.. i.e.. Activity B can start only after Activity A is completed. So Activity B starts on the 11th day.
Situation 1: You started Activity B on the 9th day, Activity A is still running
In the above situation, Activity A is not yet completed, but you start Activity B on the 9th day itself, instead of the 11th day(Activity A will complete on the 10th day). Here there is an overlap between the activities A & B. Here, we can say that start Activity B advanced by 2 days. The amount of overlap in duration or the amount of time Activity B ( successor) advanced with respect to activity A ( Predecessor) is called LEAD TIME.
Situation 2: You finished Activity A on 10th day, but you can start Activity B only on 14th.
In the above situation, Activity A completed as scheduled, but there is a delay or waiting period of 3 days for the Activity B to start. This delay is called LAG TIME.