Critical Path

The critical path is the longest sequence of dependent tasks through a project schedule — it determines the shortest possible project duration, because delaying any task on it delays the whole project by the same amount. The Critical Path Method (CPM) is the technique for computing it. Critical Chain covers a different, resource-focused scheduling method that starts from the same network diagram but manages it differently.

Building the network

Every task needs a duration and its dependencies (which tasks must finish before it can start). Drawn as a network diagram (nodes = tasks, arrows = dependencies), multiple paths typically exist from start to finish — the critical path is whichever one is longest in total duration, not necessarily the one with the most tasks.

Forward pass and backward pass

• Forward pass — starting from the project start, compute each task's earliest start (ES) and earliest finish (EF) by walking forward through the network: EF = ES + duration, and a task's ES is the latest EF among everything it depends on.
• Backward pass — starting from the project's overall finish (the latest EF computed above), compute each task's latest finish (LF) and latest start (LS) by walking backward: LS = LF − duration, and a task's LF is the earliest LS among everything that depends on it.

Float (slack)

Total float = LS − ES (equivalently LF − EF) — how much a task can slip without delaying the overall project finish. A task with zero float is on the critical path; any delay to it delays the project by the same amount. A task with positive float has room to slip (or to release its resources temporarily) without affecting the end date — but only up to that float amount, and using up one task's float can reduce or eliminate float on other tasks sharing the same downstream dependency.

Worked shape (conceptual)

Task A (2 days) → Task B (3 days) → Task D (2 days)   = 7 days, one path
Task A (2 days) → Task C (5 days) → Task D (2 days)   = 9 days, another path

Here A→C→D (9 days) is the critical path; A→B→D is 2 days shorter, so Task B has 2 days of float. Speeding up Task C (or shortening any task actually on the critical path) shortens the project; speeding up Task B does nothing to the overall finish date until B's path becomes the longer one.

Why this matters for prioritizing schedule attention

The most common practical error is spending management attention uniformly across every "urgent-looking" task rather than specifically on critical-path tasks — a task can look urgent (a tight deadline, a vocal stakeholder) while having substantial float, and a genuinely schedule-threatening task can look unremarkable until its float runs out. Crashing (adding resources to shorten a task) or fast-tracking (running normally-sequential tasks in parallel) only shortens the project when applied to tasks actually on the critical path — doing either to a non-critical task burns cost or risk for zero schedule benefit.

Common pitfalls

• Confusing "looks urgent" with "is critical" — see above; float, not perceived urgency, is what determines whether a task threatens the project end date.
• Not recalculating the critical path as the project progresses — the critical path can shift over time (a previously-critical task finishes early, or a previously-slack task's float gets consumed); treating the original schedule's critical path as permanently fixed misses this.
• Crashing a non-critical task to "make progress faster" — adds cost without shortening the overall schedule, since the critical path is unaffected.
• Ignoring resource contention in the network diagram itself — pure CPM assumes unlimited resources for scheduling purposes; if the same specialist is needed on two "parallel" critical-path-adjacent tasks at once, the network diagram alone won't surface that conflict — this is exactly the gap Critical Chain's method is built to address.

Learn more

• PMI: PMBOK Guide — CPM as part of standard schedule-management practice.
• Critical Chain for the resource-buffer-based alternative scheduling method.
• Project Management for the broader schedule/cost/scope context CPM fits into.