Critical Chain

Critical Path considers task dependencies. Critical Chain considers task dependencies and resource dependencies.

Critical Path (ignores resources):
 
Task A ──────┐
             ├──→ Task C
Task B ──────┘
 
Duration: max(A, B) + C
 
 
Critical Chain (same person does A and B):
 
Task A ──────→ Task B ──────→ Task C
 
Duration: A + B + C

Critical Path gives you an optimistic fiction. Critical Chain gives you what will actually happen.

1. Identify — Find the critical chain: the longest path through both task and resource dependencies.
2. Exploit — Strip hidden safety from estimates. Eliminate multitasking on critical chain tasks. Never let critical chain resources wait for inputs.
3. Subordinate — Everything else serves the chain. Feeding work delivers before it’s needed. Resources prioritize chain work over everything.
4. Elevate — Add resources, parallelize, cut scope. Only after exploitation is maxed out.
5. Repeat — The chain shifts. Back to step 1.

Instead of hiding safety in each task, consolidate it:

• Project Buffer — End of the critical chain. Protects the delivery date.
• Feeding Buffer — Where non-critical paths join the chain. Protects against delays in feeding work.
• Resource Buffer — Alerts key resources their task is coming. Not time, just a heads-up.

Same total safety time, dramatically different outcomes.

Three reasons distributed safety fails:

Statistical aggregation. Uncertainties don’t all materialize. Pooled buffers let variations cancel out. A 10-day project buffer protects better than 10 × 1-day task buffers. Insurance math.

Behavioral effects. Parkinson’s Law: work expands to fill available time. Student Syndrome: people delay until pressure builds. Built-in safety gets consumed whether needed or not. Consolidated buffers are only touched when actually required.

Visibility. Distributed safety hides problems until too late. Consolidated buffers give you “60% consumed with 40% done” — a clear signal to act.

Starting late is better than starting early. Counterintuitive, but the math checks out.

Early start problems:

• Student Syndrome means early start just moves procrastination earlier
• Parkinson’s Law means extra time gets used whether needed or not
• Early finishes never get passed on — people polish, delay reporting, or start something else
• More WIP means more context switching and resource collisions

Late start with buffers:

• Minimizes work-in-progress
• Reduces resource contention
• Avoids premature work on requirements that might change
• Buffer provides protection without the behavioral problems

The rule: start as late as possible while buffers absorb variation. Starting early feels safer. Starting late with buffers is safer.