Critical Chain

Tuesday, September 18th, 2007

As I mentioned earlier, when I read Kevin Fox‘s Blue Light anecdote, it spurred me to go back and read some old Goldratt books I hadn’t read yet, including his third business novel, Critical Chain.

Critical Chain doesn’t look at production or logistics but at project management.

Modern project management goes back to the 1950s, when Booz-Allen & Hamilton developed the Program Evaluation and Review Technique, or PERT, with Lockheed for the Polaris missile submarine program, and DuPont developed the Critical Path Method, or CPM, with Remington Rand for plant maintenance projects.

The basic idea behind these methods is to diagram out the various tasks within the larger project, along with their interdependencies and durations.

Two pseudo-tasks, start and finish, make the analysis clearer but don’t represent real work.

The first step in the formal analysis is to compute the early start and early finish dates for each task — the earliest it could start, given all its dependencies, and the earliest it could then end, given its duration — starting with the start pseudo-task and working to the right.

The second step in the formal analysis is to compute the late start and late finish dates for each task — the latest it could finish, without delaying the larger project, and the latest is could then start, given its duration — starting from the finish pseudo-task and working backward to the left.

Once you’ve done all that — or have made a computer do all that — you can see which tasks are on the critical path, with no slack. Any delays to any task on the critical path will delay the larger project. Any delays to any task not on the critical path will not delay the larger project — until all the slack for that task gets used up.

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