What Is the Critical Path Method (CPM)?
The critical path method is a scheduling technique that identifies the longest chain of dependent activities in a project — the chain that governs its overall duration.
The idea
The project is modelled as a network of activities with dependencies between them (columns are not cast before footings). Calculating the earliest start and finish for each activity in a forward pass, and the latest start and finish in a backward pass, reveals the float on every activity. The activities with zero float form the critical path, where any delay delays the whole project.
Why it matters to the contractor
It sets management priorities: materials and crews for critical activities must not slip, and the analysis of any obstruction starts from its position in the network. In claims, proving that a delay fell on the critical path is a condition of entitlement to an extension of time.
How to apply it
1) List the activities from the scope of work (the WBS helps). 2) Set the logical dependencies FS/SS/FF. 3) Estimate durations from quantities and productivity rates. 4) Run the network and extract the critical path. 5) Update it regularly with actual progress — the critical path shifts during execution.
Worked example
A typical critical path for a villa: excavation → footings → neck columns → ground beams → ground-floor columns → ground-floor slab → … External finishes may hold two weeks of float, so a 10-day delay there does not move handover — while a 3-day delay to the ground-floor slab delays the project by a full 3 days.
FAQ
What is the difference between CPM and PERT?
CPM uses a single fixed duration for each activity; PERT uses three estimates (optimistic, most likely, pessimistic) to treat uncertainty statistically. In construction, CPM prevails.
How many critical paths can there be?
There may be several critical or near-critical paths (1–3 days of float), and in practice they all deserve the same attention.
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