How to Make Things Faster

Chapter 39. Critical Path

Here’s a Gantt chart that depicts the execution of three tasks:

Tasks A and B can execute in parallel, and task C can’t begin until both A and B are finished. The highest-priority symptom for this business is to get C to finish earlier. So, quick quiz: how much earlier would C finish if you could make A finish twice as quickly (that is, in half the time) as it does now?

Well, if A and B execute independently, then improving the speed of A won’t make C finish any earlier at all:

A Gantt chart analyst would say that improving the duration of A will not change the time at which C completes. This is because A is not on the sequence of tasks determining the minimum time needed for an operation, called the critical path.

And it makes sense. If A and B are independent,¹ then the only way to make C finish earlier is to reduce the duration of either B, or C itself. If you improve B so much that it runs in less time than A, then A will replace B on the critical path and become the focus of your attention. Understanding your critical path helps you make sure you’re always working on the right problem.

¹ The assumption that A and B execute independently may or may not be realistic. Imagine, for example, that B uses a lot of a resource for which A creates strenuous ...