Glossary
Critical Path vs Critical Chain
Critical Path Method (CPM) sequences activities by logic dependencies. Critical Chain Project Management (CCPM) extends CPM by also constraining for resource dependencies, treating shared resources as the binding factor.
Critical Path Method (CPM) vs Critical Chain Project Management (CCPM).
| Critical Path Method (CPM) | Critical Chain Project Management (CCPM) | |
|---|---|---|
| Origin | Du Pont / Remington Rand, 1950s | Eliyahu Goldratt, Critical Chain (1997) |
| What determines the longest sequence? | Logic dependencies between activities (resources assumed unlimited) | Logic dependencies + resource dependencies (shared resources count) |
| Activity duration estimating | Typically padded — each estimate carries individual contingency | Stripped to ~50% confidence; padding aggregated as buffers |
| Buffer placement | Implicit, inside each activity duration | Explicit and aggregated — project buffer at end + feeding buffers at chain joins |
| Resource levelling | Separate operation, applied after CPM logic | Built into the methodology — resource conflicts are part of chain identification |
| UK infrastructure adoption | Universal — NEC4, P6, MS Project, MoD, all UK majors | Rare — occasional pharma/R&D use; almost absent in construction |
| Tool support | Native in P6, MS Project, Asta, Open Plan, Deltek | Specialist tools (Concerto, Lynx, ProChain); minimal native CPM-tool support |
| Practical takeaway for CPM users | — | Buffer aggregation idea is worth borrowing — many UK programmes now use programme-level contingency rather than activity-level padding |
The Critical Path Method (CPM), developed in the 1950s, identifies the longest sequence of dependent activities through a project network. The critical path determines the project end date — any delay on it pushes completion. CPM treats activities as logic-linked but assumes resources are unlimited; if two parallel activities both need the same crane, CPM doesn't notice. The planner must apply resource levelling separately and accept that the levelled schedule may differ substantially from the unlevelled critical path.
Critical Chain Project Management (CCPM), introduced by Eliyahu Goldratt in his 1997 book of the same name, addresses the resource-availability gap directly. The critical chain is the longest path through the project after both logic dependencies AND resource dependencies are accounted for. Two activities that aren't logically dependent but share a single critical resource (one welder, one crane) are treated as effectively sequential — and that constraint may produce a critical chain longer than the classical critical path.
CCPM also reframes how buffer is managed. Rather than padding individual activities with hidden contingency (which CCPM theory says invariably gets consumed via Parkinson's Law and student-syndrome behaviour), CCPM strips activity estimates to ~50% confidence and aggregates the buffer at the end of feeding chains and at the project finish. The argument is that a single shared buffer absorbs variability more efficiently than many small individual buffers — a defensible statistical position, though one that requires significant cultural adjustment to implement.
In UK practice, CPM remains the dominant approach: NEC4, P6, MS Project and the entire UK infrastructure controls profession are CPM-native. CCPM appears occasionally in pharmaceutical, R&D and aerospace contexts but is rare on construction and infrastructure. The most useful CCPM idea for CPM practitioners is the buffer concept — many sophisticated UK programmes now use a hybrid: CPM logic + aggregated programme-level contingency rather than activity-level padding.
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Frequently asked
- What is the difference between Critical Path and Critical Chain?
- The Critical Path is the longest sequence of activities in a project network when only logic (sequence) dependencies are considered — resources are assumed unlimited. The Critical Chain is the longest sequence when both logic AND resource dependencies are considered. If two parallel activities share a single crane, the Critical Path treats them as parallel; the Critical Chain treats them as effectively sequential because the crane can only be in one place at a time.
- Is Critical Chain better than Critical Path?
- Neither is strictly better — they answer different questions. CPM is the universal framework in UK infrastructure (NEC4, MoD, IPA, P6, MS Project) and is what your client and contractor will expect. CCPM has theoretical advantages around buffer management but requires a significant cultural shift and tools that don't fit the standard UK toolchain. The most pragmatic answer is to use CPM as the operating framework and borrow CCPM's buffer-aggregation idea (programme-level contingency rather than activity padding) where it fits.
- Where does Critical Chain handle buffer differently?
- Classical CPM allows padding inside each activity duration estimate — every planner adds their own contingency. CCPM theory argues this is wasteful because (a) Parkinson's Law: work expands to fill the time available, so padding gets consumed; (b) student syndrome: people start late if they have buffer, then run into the wall; (c) summing N individual safety margins is statistically less efficient than aggregating one project-level margin. CCPM strips activity estimates to a 50%-confidence median and places explicit buffers at chain joins and project end, where they act as a shared shock absorber.
- Do UK NEC4 contracts use Critical Path or Critical Chain?
- Critical Path. NEC4 Clause 31 (Accepted Programme requirements), the compensation event mechanism, and the assessment of delay all assume CPM as the underlying scheduling methodology. The Accepted Programme is a CPM logic network. Float (free, total, terminal) is a CPM concept. While there is nothing in NEC4 that explicitly forbids CCPM, every UK construction lawyer, contract administrator, and IPA reviewer expects to read a CPM schedule.
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