When should I use LAG in MS Project?

The definition of lag is the amount of time a successor task must be delayed. If you’ve attended any of my classes where I walk my students through activity sequencing, you’d already know that I like using two-word definitions and that I simply refer to lag as a forced wait.

Let’s walk through an example of when lag makes sense. I am redecorating a room and part of this project includes painting the room and hanging up pictures. Therefore, Paint walls is a predecessor of Hang up pictures. If I intend to “force” a waiting time between the tasks (I should probably let the paint dry a bit!), I place lag right on that finish-to-start dependency. If I want to let the paint dry for a day, then I’d add 1 day lag. The start of the successor task will now begin after a one day waiting period once the predecessor has finished. Lag is dynamic so if it takes more or less time for me to paint, the lag will still remain as a fixed 1 day wait prior to hanging up pictures.

An alternative to adding lag is creating a new task, such as Wait for paint to dry. The reason why lag is usually the preferred method is because there is no effort for paint drying. Unless you need to exhibit these types of tasks on the Gantt Chart, then just use lag and save yourself the trouble. In general, using lag will reduce the number of tasks you need to view on your project schedules.

Of course, if you’re not using dependencies at all, then you can’t actually apply lag!