Critical path method (CPM)

Critical path method (CPM) is a scheduling technique that finds the longest chain of dependent tasks in a project. That chain sets the shortest possible finish date, and any task on it has zero float, so a day lost is a day added.

CPM is arithmetic, not management philosophy. You list the tasks, give each one a duration, record what must finish before it can start, then add up the chains. The longest chain is the critical path.

It sits at the opposite end from continuous delivery. CPM assumes a fixed scope and finds the longest chain of dependent tasks. Continuous delivery assumes scope keeps arriving and optimises the pipeline instead.

Key takeaways

• The critical path is the longest chain of dependent tasks, and it sets the earliest date a project can finish.
• Float is the slack a task has before it delays something else. Tasks on the critical path have zero float.
• Shortening a project means shortening the critical path. Adding people to a task with float changes nothing.
• CPM assumes a known scope, which is why it fits build projects better than continuous product work.

How it works

Build the task list, give every task a duration, and record its dependencies. Add the durations along each possible chain from start to finish. The longest chain is the critical path, and every task on it has zero float.

Here is a small network for a software cutover, in working days.

Read it as two chains. A to B to D to F runs 24 days, so it is the critical path. A to C to E to F runs 13 days, which leaves C and E with 11 days of float each.

That float is the whole point. You can start the branding late, or lose a trainer for a fortnight, and the go live date does not move.

Delay data migration by three days, though, and the project finishes three days later. There is nothing in the chain to absorb it.

Float is not a buffer you are allowed to spend. It is a by-product of the network — it vanishes the moment a task with float slips far enough to join the critical path.

CPM does not tell you what to do about a late task — it only tells you which late tasks matter. That is a smaller claim than most scheduling software implies, and a more useful one.

A Gantt chart draws the same data as bars on a calendar. CPM is the calculation that tells you which of those bars you cannot afford to move.

The technique travels well across contracts. On project outsourcing work, one shared critical path gives client and vendor a single agreed list of the tasks that control the date.

PMO outsourcing buys the discipline itself. Someone external maintains the network, chases durations and reissues the path whenever a dependency changes.

A program manager watches several of these paths at once, because one team’s slipped dependency is often another team’s critical task.

An IT project manager usually keeps the network in software, though the arithmetic is simple enough to check by hand — and checking by hand is how optimistic durations get caught.

Estimates are only as reliable as the schedule beneath them. The US Government Accountability Office publishes guidance meant to help agencies produce reliable, high quality cost estimates, and a dated task network is what makes those numbers defensible.

You do not need to buy a methodology to learn this. NASA’s systems engineering handbook, which documents how the agency plans complex technical work, is published free to read.

Examples

CPM earns its keep wherever tasks physically cannot overlap. Construction, migrations and site launches all have chains where one thing must finish before the next can start, and no amount of extra staff changes that order.

Construction fitout. Power before plasterboard, plasterboard before paint, paint before inspection. Paint usually carries float; the electrical inspection almost never does.

ERP cutover. Data migration sits on the critical path in nearly every system replacement, while user training runs alongside it and only becomes critical in the final week.

Offshore site launch. Standing up a delivery centre in Manila or Bogotá runs lease, then fitout, then internet provisioning. Recruitment runs in parallel and rarely touches the path.

Regulatory deadlines. A fixed legal date turns a soft plan into a dated network, and CPM shows which two or three tasks deserve the extra money.

Software helps once the network passes about 50 tasks. Job scheduling software recalculates the path whenever a duration changes, which beats redrawing a chart by hand.

Two things break CPM in practice — durations supplied by the person who will be blamed for them, and dependencies nobody wrote down.

So ask for a range, not a number. A task quoted at eight days is usually six to twelve, and that spread decides whether your path holds.

Related terms

• Job scheduling software: the tooling that recalculates a critical path as durations change.
• Project coordinator: the person who collects the actual durations the calculation depends on.
• Gantt chart: the bar view of a schedule; CPM is the maths that ranks the bars.
• Program manager: coordinates several projects, each with a critical path of its own.
• PMO outsourcing: buying the function that maintains schedules and dependencies for you.

FAQ

What is the critical path method?

It is a scheduling technique that adds up task durations along every chain of dependencies and picks the longest one. That longest chain sets the earliest possible finish date.

What does float mean?

Float is how long a task can slip before it delays something downstream. A task with 11 days of float can start late and still cause no harm.

Can a project have more than one critical path?

Yes. When two chains have the same total duration, both are critical, and a delay on either one moves the finish date.

How is CPM different from a Gantt chart?

A Gantt chart is a picture of the schedule. CPM is the calculation that identifies which tasks in that picture have zero float.

Does CPM still matter with agile delivery?

It matters wherever scope is fixed and dependencies are physical, such as migrations, launches and construction. It fits continuous product work poorly, because the scope keeps changing.

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