The critical path is the longest path through your project schedule. It tells you the shortest time the project can take, and which activities can't slip without delaying the end date. PMP questions test whether you can find it, work out float, and choose the right way to shorten a schedule.
The key terms
- Critical path: the sequence of activities with the longest total duration. Its activities usually have zero float.
- Total float: how long an activity can be delayed without delaying the project end date. Total float = late start − early start (or late finish − early finish).
- Free float: how long an activity can be delayed without delaying the early start of the next activity.
- Forward pass: working left to right to find the early start (ES) and early finish (EF) of each activity.
- Backward pass: working right to left to find the late start (LS) and late finish (LF).
Worked example
A small software release has five activities. Durations are in days:
| Activity | Duration | Depends on |
|---|---|---|
| A: Design | 6 | Start |
| B: Build back end | 11 | A |
| C: Build front end | 7 | A |
| D: Write user guide | 2 | C |
| E: Test and release | 5 | B and D |
Step 1: List the paths
- A → B → E = 6 + 11 + 5 = 22 days
- A → C → D → E = 6 + 7 + 2 + 5 = 20 days
The longest path, A → B → E, is the critical path. The project can't finish in less than 22 days. (If two paths tie for longest, the project has two critical paths, which means more schedule risk.)
Step 2: Forward pass (early dates)
- A: ES 0, EF 6
- B: ES 6, EF 17
- C: ES 6, EF 13
- D: ES 13, EF 15
- E: ES 17 (it waits for B, the later of B and D), EF 22
Step 3: Backward pass (late dates)
- E: LF 22, LS 17
- B: LF 17, LS 6
- D: LF 17, LS 15
- C: LF 15, LS 8
- A: LF 6, LS 0
Step 4: Float
- A, B and E: total float = 0. They're on the critical path.
- C: total float = 8 − 6 = 2 days
- D: total float = 15 − 13 = 2 days. Free float is also 2 days, because E can't start before day 17 anyway.
So the front-end work and the user guide can slip by up to 2 days in total without moving the release date.
How to shorten the schedule
If the sponsor needs the release sooner, only shortening the critical path helps. Two techniques come up constantly:
- Crashing: add resources to critical activities, such as a second back-end developer on B. It costs money but adds little risk. Crash the cheapest days first.
- Fast tracking: run activities in parallel that were planned in sequence, such as starting testing before the build is finished. It costs nothing extra but increases the risk of rework.
Watch out: once you shorten B by more than 2 days, the other path becomes critical too. Always recheck the paths after a change.
Exam tips
- A delay on a critical activity delays the project by the same amount. A delay within an activity's float doesn't.
- Adding people to activities that aren't on the critical path doesn't shorten the project.
- If the question says "no extra budget", think fast tracking. If it says "risk must stay low", think crashing.
Practise it
Read the free Plan and Manage Schedule chapter, use the PMP formulas cheat sheet, and test yourself with the free PMP practice questions. In PMP Quest, the schedule level puts you in exactly these situations.