Reading a CPM schedule: critical path, float and the data date

A critical path method (CPM) schedule can run to thousands of activities, and most people on a project only ever see a filtered slice of it. Three ideas explain nearly everything on the page.

The critical path

Every activity has a duration and a set of relationships: this one can't start until that one finishes. Follow those relationships from the first activity to the last and you get many possible paths through the job. The longest one is the critical path. It sets the earliest possible completion date, and a one-day delay to any activity on it is a one-day delay to the project.

That is why the critical path gets its own color on a bar chart. It tells the team where a problem costs calendar time rather than just effort.

Float

Activities off the critical path have float: the number of days they can slip before they push something that matters. An activity with twenty days of float is not urgent this week. An activity with two days of float deserves a phone call.

Float belongs to the project, not to a single trade. When one subcontractor quietly uses all of it, the activities behind them inherit a critical path they didn't plan for. Watching float shrink from update to update is one of the earliest warnings a schedule gives.

The data date

The data date is the line drawn through the schedule on the day it was updated. Everything to the left is recorded: actual starts, actual finishes, percent complete. Everything to the right is a forecast, recalculated from what really happened.

A schedule without a current data date is a plan. A schedule with one is a measurement.

Using the three together

In a weekly meeting the useful questions follow directly:

• Did anything on the critical path fail to start or finish as planned since the last data date?
• Which near-critical activities lost float, and why?
• What has to be released, approved or delivered in the next three weeks to keep those activities moving?

A schedule read this way becomes a short list of decisions.