Critical Path Method

Definition

PERT/CPM models a project as a network of activities, each with a duration and a list of immediate predecessors that must finish before it can start. Adding up the longest chain of dependent activities from start to finish gives the project's minimum possible completion time, and that chain is the critical path: every activity on it is "critical" — any delay to a critical activity delays the whole project — while activities off the critical path have slack, meaning they "can be delayed... before they cause an increase in the total project completion time" without changing the finish date.

In the Book

Chapter 9 works the method through the Western Hills Shopping Center expansion project, listing activities such as "Prepare architectural drawings" (activity A, 5 weeks, no predecessor), "Identify potential new tenants" (activity B, 6 weeks, no predecessor), "Develop prospectus for tenants" (activity C, 4 weeks, depends on A), and "Select contractor" (activity D, 3 weeks, depends on A), building out a full activity-predecessor-duration table before constructing the network diagram and computing the critical path and project completion time. The book distinguishes PERT (developed for the Navy's Polaris missile project, built to handle activities whose durations were genuinely uncertain) from CPM (developed at DuPont for industrial projects with known activity times and known cost/time trade-offs for compressing them), and revisits the same shopping-center project later in the chapter under uncertain activity times to show how probabilistic durations change which path is likely to be critical.

Why It Matters

Not every task in a complex project matters equally to the deadline — the critical path identifies exactly which activities the project manager must protect and which have room to slip without consequence, replacing a generalized sense of "everything is urgent" with a specific, defensible answer about where schedule risk actually concentrates. That distinction — a small subset of a system's parts that fully determines its overall throughput time, with everything else carrying slack — is the same structural insight that recurs anywhere dependent tasks compete to determine an aggregate deadline.