Setting up dependencies, or task relationships, is an integral part of working in Project. But many newer users ask, Which relationship should I use, and What are these “lag” and “lead” things about?
The choice of dependencies, as well as using the two other items, derives from the nature of the tasks. This is the one thing you have to bear in mind, because there’s no all-inclusive formula for determining these. That said, there are a couple of general pointers you can use to figure it out.
If Task A has to be completely finished before Task B can start, such as demolishing a house before building a new one, the relationship will be Finish-to-Start.
Say you’re writing a book and want to make sure the illustrator gets started more or less immediately along with the writing. A Start-to-Start relationship might be in order. The artwork can happen pretty quickly once you have a decent chunk written.
If you’re building a house or office, the framing has to be complete before the plumbing and electrical, the drywall, etc., can be finished. So a Finish-to-Finish relationship would work for this.
(The fourth type, Finish-to-Start, is so rarely used I won’t go into it here. Suffice to say in the twenty-plus years I’ve taught the program, I’ve seen maybe two occurrences of it. No offense.)
Okay so far. Now, what about lag and lead?
Those come into play when two tasks have to have a time gap between them (lag), or can partly overlap (lead); the dependencies still matter, but this is something else. Start-to-Start and Finish-to-Finish can overlap entirely, if the nature of the tasks allow, but lag and lead usually come in with a Finish-to-Start. (By the way, it’s pronounced “leed”, as in “lead or follow”, not “led”, as in a pencil.)
If you pour concrete, even the quick-setting stuff needs a few days, minimum, to be ready to walk on etc. A lag—a delay—would go in there to indicate a setting time is required regardless. You could include it in the task time, but the lag makes the situation clearer to anyone who looks.
In the book-writing example, if the author wants to get about a third of the book written before the illustrator starts sketching, it might help the artwork to have a solid direction to go in. So a Start-to-Start with a lag, or a Finish-to-Start with an overlap, or lead (sometimes known as negative lag) would be the natural thought here. (And the lag—or lead—can be written as a percentage of the predecessor task’s time, which is helpful as it doesn’t require one to do the calculation in one’s head.)
As you can see, the nature of the work, and of the tasks, and their “natural” relationship, will often determine how one hooks them together in Project. I’ve often pointed out that it can help to work on a legal pad and pencil or pen first—some people find figuring dependencies on paper before sitting at the computer can make it easier.