The critical path in plain language
On a four-storey clinic, the landscaping crew is two weeks behind and nobody in the office seems worried. Then the elevator rails arrive two days late, and the project manager calls you within the hour. Your new assistant superintendent wants to know why one slip matters and the other does not.
Most activities wait for others to finish, so a schedule is really a network of linked chains. The critical path is the longest chain through it, and its length is the shortest time in which the job can be built: a delay to any activity on it delays completion unless the time is won back later. Activities off the path have float, so they can slip for a while without moving the finish. Crews usually see only their own bar, which is why a two-day slip on the path surprises everyone except the scheduler.
Finding the path on your job
- Start at the finish date and walk backwards: of the activities that must end before completion, find the one that ends last, then do the same for that activity, and so on.
- Check that every activity is linked to something before and after it. An activity with no predecessor or successor cannot show whether it is critical, so have the logic fixed before you trust the path.
- Read total float. Critical activities normally have the lowest: zero when the schedule is calculated to its own finish, below zero when float is measured to a fixed contract date the job will miss. A job can have more than one critical path.
- Name the controlling activity — the first unfinished activity on the critical path — and tell the crew doing it.
- Watch chains with only a few days of float. One bad week can make them critical.
- Mark critical activities on the three-week look-ahead so every foreman can see which handoffs cannot wait.
Common mistakes
- Calling the longest single activity critical. The path is the longest chain of linked activities, not the biggest bar.
- Assuming the path stays where the baseline put it. It moves as work finishes early or late, so read it again at every update.
- Holding dates in place with date constraints instead of logic. A constraint overrides the links between activities, and the schedule stops showing the real path.
Checklist
Checking the critical path this week
- Forecast finish, compared with last week
- The controlling activity, named
- Critical activities for the next three weeks, marked on the look-ahead
- Chains with only a few days of float, listed
- Unlinked activities and date constraints, sent back to the scheduler
Check your understanding
Framing on level 3 slips four days, and the update showed it had twelve days of total float. Does the finish date move?
