A resource assigned to tasks that overlap might become overallocated during the overlap period. Tasks that have a start-to-start link and tasks with lead time are two kinds of tasks that can have overlap time.
If one of the tasks can start later without making your schedule unacceptably long, you’re in luck. You can resolve the overallocation by delaying the task.
Delay is an extra amount of time you can add between overlapping tasks to ease scheduling constraints for an overallocated resource. When you add enough delay time to make the second task start after the first task finishes, you fix the problem. The overallocated resource is no longer scheduled to work on both tasks at the same time.
Delay has the same effect as lag time, but you use each for different purposes. You add lag time between tasks to show when those tasks will actually be accomplished. For example, the exhibit planner might add lag time between painting a wall and hanging clocks on the wall, because the planner knows the paint will take time to dry. You add delay between tasks specifically to resolve a resource overallocation.
Though you can’t see any difference in the Gantt Chart view between lag time and delay — they both increase the gap between task start dates — Microsoft Project tracks each separately. For example, if you add 2 days of lag time between two tasks and then add 1 day of delay, Microsoft Project postpones the start of the second task from 2 days to 3 days, but still shows only 2 days of lag time in the Lag field and 1 day of delay in the Leveling Delay field.
Microsoft Project provides two ways to add delay time to a task. You can add delay to one task at a time or allow Microsoft Project to add delay times to all tasks with overallocated resources all at once.