Time-cost trade-off

Most projects can be finished sooner by spending more: extra shifts, more crews or equipment, faster methods or premium materials. Crashing is the systematic way to decide which activities to speed up and by how much, to shorten the project at least extra cost. It is used when a client needs early completion, when delays must be recovered, or when bonuses or penalties change the economics.

Each activity has a normal duration and cost, and a crash duration and cost, the shortest possible time at a higher cost. The cost slope is the extra cost per day saved. Direct costs rise as the project is compressed, while indirect costs, such as site overheads, supervision and equipment hire, fall with shorter duration. The total cost curve, the sum of both, has a minimum at the optimum duration.

The procedure is iterative. Find the critical path. Crash the critical activity with the lowest cost slope by one day, or by the amount possible before another path becomes critical. Recompute. When there are multiple critical paths, each must be shortened together, so look for the cheapest combination, or a single activity common to all paths. Stop when the extra direct cost exceeds the saving in indirect cost, or when no further crashing is possible. Fast-tracking, overlapping activities planned in sequence, is the other compression tool; it costs less money but adds risk of rework.