Microsoft Project encourages you to jump straight to the Gantt chart, without bothering with post-it notes and critical paths first.
Usually when people are doing this they make a mini-Gantt for each department, and then combine them all, like this:
So they have several linear paths, all in parallel.
They forget that actually some of the tasks might be held up waiting for the other departments:
Basically there are bound to be some cross-connections, for example like this:
You could say that if you were showing this project plan as dependencies using post-its, the project is being planned wrongly like this:
… is is bound to be WRONG; there are always crosslinks between the departments.
The diagram is much more likely to look like this, if drawn correctly:
And since we don’t want arrows going backwards we need to spread it out a bit so it looks like this:
Which leads to this critical (longest) path:
So here are the critical tasks, the ones you really need to focus on, shown in red
These critical tasks were totally missed on the original simplistic-and-wrong diagram!
And the best way to work all this out is by doing the post-its first. Jumping straight to the Gantt, as encouraged by MS Project, makes it very hard to work out where the cross links go, and in fact they are usually ignored or forgotten, thus affecting the critical path, giving an incorrect total estimate of the time for the project, and meaning we don’t know which tasks are critical
So, going back to the Gantt chart, with the cross links making the total time for the project longer, there is in fact a hidden critical path, which probably jumps between departments, which really defines how long the project has to take – like this:
Ideally the critical tasks would be shown in red, like this. The critical tasks have to be watched carefully, while the floating tasks can take longer without it mattering. So everyone needs to know what’s critical:
MS Project can show the critical tasks like this, but it’s rarely done.
You can (and should) go one step further and pull out the critical path to the top, so it’s REALLY clear, like this:
So the recommended process is to do the post-its first (even if you’re using MS Project) and make sure you get all the crosslinks in, correctly.
Then you know the critical path, which you can use to easily draw your critical path (using MS Project or Excel or whatever you want to use) – and then you can add the floating (non-critical) tasks underneath.
Then we have a clear and correct critical path. We know which tasks to focus on, and we have a realistic and achievable finish date.