The critical path in plain words
The critical path is the chain of work that decides when a project ends. If any piece of work on it slips by a day, the whole project slips by a day. Other work can move a little with no effect — it has float.
An example
Take a small project of five pieces of work:
| Work | Duration | Depends on |
|---|---|---|
| A. Requirements | 5 days | — |
| B. Server development | 10 days | A |
| C. User interface | 6 days | A |
| D. Documentation | 3 days | A |
| E. Testing | 4 days | B and C |
After the requirements, three pieces run in parallel. Testing waits for both the server and the interface.
- Path A → B → E takes 5 + 10 + 4 = 19 days.
- Path A → C → E takes 5 + 6 + 4 = 15 days.
- Path A → D takes 5 + 3 = 8 days.
The longest path is A → B → E. That is the critical path: the project cannot end in fewer than 19 working days.
What float is
The interface (C) can be done 4 days later and testing will still start on time — the server holds it. Those 4 days are the float of C. Documentation has even more: 11 days.
Work on the critical path has zero float.
How to use it
- Watch critical work first. A 2-day slip on the server is a 2-day slip of the project. A 2-day slip on the interface is nothing yet.
- Watch work with little float. If the interface slips 5 days, it uses up all its float and becomes critical itself. That is why it helps to highlight work with only a few days of float.
- Speed up only critical work. An extra developer on the interface won’t bring the end date closer. On the server, it might.
- Recalculate after changes. The critical path changes as the project goes: yesterday the server was critical, tomorrow it may be the interface.
Common misconceptions
- “Critical means important.” No: critical means no slack in time. Work that matters most to the business can have plenty of float.
- “There is one critical path for good.” It changes with every shift in work and dependencies.
- “You can work it out in your head.” For five tasks, yes. For two hundred with mixed dependency types and a holiday calendar, no.
Where to do it
In Etapero the critical path is worked out from dependencies and the production calendar and highlighted on the Gantt chart, and work with little float is highlighted separately. How to build the plan itself is in “How to make a Gantt chart”.