Lessons · Project management · the critical path
The critical path: the chain with no room
The critical path is the longest path through the network and the chain of activities with zero float; its length is the project's duration, and any slip on it moves the end date.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
With thirty activities, you cannot watch them all. The critical path tells you which eight to watch every morning. The other twenty-two can slip a little; these eight cannot slip at all.
How to think about it
Do both passes, compute float, and mark every zero-float activity. They form a continuous chain from start to end; that is the critical path. Check it by adding the durations along it: the sum must equal the project duration.
Worked example
Floats: A 0, B 0, C 2, D 0From the forward and backward passes.
Critical path: A → B → DThe zero-float chain. It is continuous, from the first box to the last.
3 + 4 + 5 = 12 daysThe check: the sum along the critical path is the project duration.
C is not critical, with 2 days of floatWatch it, but watch A, B and D first.
Your turn
Write the test for whether an activity is on the critical path.
Its total float is .
Solve one, graded on the server
The trap
Believing the critical path is fixed. When C slips three days, A-C-D becomes 13 and C is now critical while B has a day of float. Recompute after every change.