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Lead Time and Lag Time
The reason we’re looking at this is it can often get confused when we’re looking looking at examples in the PMP exam. It is really good to get a good idea of it, as you will find these questions on the PMP exam, so it’s good to get your head around it first before going into the exam. Also then you can use it in your project management career.
Lead time and Lag time refers to your schedule, during your schedule analysis.
Lead time is the amount of time that the next activity can be brought forward. So two activities can be done in parallel. Lag time when we’re referring to the amount of time that the next activity will be delayed – so it’s lagging behind. Lead time we are leading it forward.
Now remember in Precedence Diagramming Method or the Critical Path Method, we can have different task dependencies such as Finish to Start, Finish to Finish, Start to Start, or Start to Finish, where for example the second activity cannot finish until the first activity has started.
Lead time only applies to our Finish to Start, where our next activity cannot start until our previous activity has finished. But lag time can affect all of these, so any of these activities can lag behind or have a lag in between those two activities.
Let’s look at an example of Lead Time.
This is a photoshoot for example, and it will take four days. Photo editing will take six days after that. Now instead of waiting until the end of the four day photo shoot to begin editing those pictures we could start editing after the first day of shooting. Then instead of 10 days in total (because we’ve got four days photographing and six days editing) we’re bringing the photo editing forward, we’re leading it forward, and using that lead time that’s available. Now our total time sits at 7 days instead of the 10 days, because we’ve led it forward and we’ve taken advantage of that.
As you can see, the two items are now done in parallel, they’re done at the same time for a portion of those activities.
Lag is moving it the other way. Let’s say for a house, a house frame might take five days to put up but it also has to wait five days after the concrete foundation has been laid – we don’t want to put anything on that concrete foundation in case it messes with the integrity of that concrete foundation for example, and we just want to make sure that it sets properly. So we have to have a lag in our project schedule, that’s the five day lag for our concrete to set. Our second activity here is lagging behind the first activity and that lag in this case is five days long.
That is the idea of leads and lags in your project schedule.
– David McLachlan