Leads and lags are terms that are closely related to the relationship that can happen for multiple scheduled activities within a project management strategy. The former is referred to as the acceleration of the successor activity. This means that the second task can commence in parallel with the first task. Lead compresses the combined duration of the two running activities. The latter, on the other hand, is the delay of the successor activity and it is the time that needs to pass before the next task can commence. This means that one task should be completed first before the other begins. The time in between is considered the lag.
Applying leads and lags is very important in the management of the project schedule. In a project management network, schedule dependencies vary and leads and lags are used to manage them. The four types of dependencies include (1) finish to start, (2) finish to finish, (3) start to start and (4) start to finish.
These dependencies can be changed by applying leads and lags and this allows project managers to determine which tasks they can start in parallel with the active tasks and which ones they have to put on hold until after the existing tasks have been completed.
This term is defined in the 5th edition of the PMBOK.