Critical path method (CPM)
Also: Critical path, Total float, Float, Slack
Quick definition: A scheduling technique that models a project as a network of dependent activities and identifies the longest path through it — the critical path — which sets the shortest possible project duration.
Explanation
Beginner explanation
Some tasks can only start after others finish. CPM chains them together and finds the sequence that takes longest. Any delay on that chain delays the whole project; tasks off the chain have some spare time, called float.
Technical explanation
Each activity has a duration and logical relationships (finish-to-start, start-to-start, finish-to-finish, start-to-finish, with optional lags). A forward pass computes early start and finish dates; a backward pass computes late dates. Total float is the time an activity can slip without delaying project completion; activities with zero (or the lowest) total float form the critical path. Free float is the slip possible without delaying any successor.
CPM schedules are the contractual baseline on most commercial and public projects and the basis for delay analysis. In VDC practice, linking CPM activities to model elements produces a 4D simulation for sequencing and logistics.
Practical example
On a mid-rise residential project, the critical path runs through foundations, the concrete frame, the building envelope and elevator installation. Interior finishes on lower floors carry 15 working days of float. When the window supplier is three weeks late, the envelope slips, the critical path moves, and the contractor must recover time or seek an extension.
Why it matters
CPM shows which activities actually drive the end date, so managers focus effort there. It is also the basis for justifying or rejecting time-extension claims.
Common confusion
A Gantt (bar) chart is a way of displaying a schedule; it is a CPM schedule only if the bars are linked by logic. A Look-ahead schedule is a short-interval plan derived from the CPM schedule, not a replacement for it.
Historical context
CPM was developed in the late 1950s by Morgan R. Walker of DuPont and James E. Kelley Jr. of Remington Rand, and presented in their 1959 paper "Critical-path planning and scheduling". PERT was developed at about the same time for the US Navy.
Standards and references
- Critical path method. Wikipedia.Historical record
Supports: Origin with Walker (DuPont) and Kelley (Remington Rand); 1959 Kelley and Walker paper; PERT developed in parallel; total float concept - A Guide to the Project Management Body of Knowledge (PMBOK Guide), Eighth Edition. Project Management Institute (PMI), 2025.Widely accepted professional interpretation
Supports: Schedule as a project management performance domain · Status: Published November 2025; six principles and seven performance domains (governance, scope, schedule, finance, stakeholders, resources, risk).
VDC Stack.io writes its own explanations and does not reproduce copyrighted standards text. Consult the cited standard for normative wording.
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VDC Stack.io. (2026). Critical path method (CPM). In VDC Stack.io Knowledge Dictionary. Retrieved from https://vdcstack.io/dictionary/critical-path-method/