Critical Path Method

Meaning

The Critical Path Method (CPM) is a network technique for scheduling and controlling a project, and of the planning and control techniques used in business it is the most widely used. It sets out a project's activities in the most logical order in which they can be done, and plans and controls the work along that order. It is meant for projects whose activity durations can be estimated with fair confidence in advance.

How CPM works

Take a contractor building a hospital wing in Bhopal. The work is first split into separate activities: site clearing, foundations, structure, wiring, plumbing and finishing. For each one the contractor settles what has to be complete before it can begin; the wiring, for instance, cannot start until the structure is up. Drawn as a network diagram, these links show at a glance which activities depend on which, and a step the plan has missed shows up as a break in the chain. CPM then uses the network to get the most out of the available time and resources: it picks out the critical activities and fixes management's attention on them, taking each activity's expected time as the time the work will actually take.

The critical path follows from this network. It is the longest chain of dependent activities running from the start of the project to its end, so its total duration is the shortest time in which the whole project can be finished. Activities on this chain have no spare time, or float: if any one of them slips, the finishing date slips with it, whereas activities off the path carry some float and can be delayed a little without harm. That is why management keeps its closest watch on the critical activities.

Merits and limitations

• Besides fixing the timetable, it gives everyone involved a common format for setting out the project's plans, schedules and costs.
• Its planning demands are heavier than usual, so planning costs more. The extra spending is justified because control is focused on the critical path alone, and the firm need not pay for tight supervision of every activity in the project. For the Bhopal contractor, that means the site engineers check the critical activities daily instead of inspecting every trade with the same intensity.
• Through better planning, organising and control of the work, it helps management complete the project economically and efficiently.

CPM and PERT

PERT and CPM are the two most popular network techniques. PERT uses probability to estimate activity times and is suited to projects whose timings are uncertain; CPM, by contrast, is used where timings are relatively well known.