Guide
The critical path method,
worked through.
The critical path method finds the longest chain of dependent tasks, which sets the earliest finish date. Find it with a forward pass and a backward pass, then any task with zero float is critical. Here is a worked example.
In development: ZeroPlan
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1. List the tasks, durations and dependencies
Write every task, how long it takes, and which tasks must finish before it can start. This example has six tasks, measured in days.
- A: 3 days, no predecessor
- B: 2 days, after A
- C: 4 days, after A
- D: 1 day, after B
- E: 3 days, after C and D
- F: 1 day, after E
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2. Forward pass: earliest start and finish
Start the first task at day 0. Each task's earliest finish is its earliest start plus its duration. A task with several predecessors starts when the last of them finishes. The last finish is the project length: 11 days.
- A 0–3, B 3–5, C 3–7, D 5–6
- E starts at the later of C (7) and D (6), so 7–10
- F 10–11: the project takes 11 days
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3. Backward pass: latest start and finish
Work back from the end. The last task's latest finish is the project length. Each task's latest finish is the earliest latest-start among the tasks that follow it, and its latest start is that minus its duration.
- F 10–11, E 7–10, C 3–7, D 6–7, B 4–6
- A must finish by 3, the earlier of B (4) and C (3), so 0–3
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4. Float, and the critical path
Total float is latest start minus earliest start: how long a task can slip without moving the finish date. Tasks with zero float form the critical path. Here that's A, C, E and F, 11 days. B and D each have 1 day of float.
- Critical path: A → C → E → F
- A delay to any critical task delays the whole project
- Recheck after every change: the path can move
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5. Let software keep it up to date
By hand this is fine for ten tasks; for a real project, scheduling software recalculates the path every time something changes. ZeroPlan, from Black Zero Labs, shows the critical path and float on the chart. It's in development and not released yet.
Apps used
The apps behind it.
The apps are designed to open the same project, so nothing has to be exported between them. You'll install only the ones you use.
Questions
Good to know.
Yes. When two chains of tasks are equally long, both have zero float and both are critical; a delay on either one moves the finish date.
Total float is how long a task can slip without delaying the project. Free float is how long it can slip without delaying the next task's earliest start. In the example, D has 1 day of both.
Spreadsheets can do it with formulas, and some project tools have free tiers. ZeroPlan will be included in every Black Zero Labs plan, including a free plan with limits, but it isn't available yet.
Early access
See the path,
before it slips.
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