CVEN9702 Chap.5 Project Compression and Cost Trade-offs
Project Compression and Cost Trade-offs
Project compression shortens duration by changing activity methods, resources or working arrangements. Only reductions on the current critical path can reduce completion. Crash slope compares additional direct cost with time saved, but the least expensive activity is useful only when its reduction is effective at project level.
Multiple critical paths require a shared activity or simultaneous reductions on every controlling branch.
What this chapter covers
- 01
Confirm which path controls before spending on compression
- 02
Calculate crash cost per unit of activity time saved
- 03
Respect normal time, crash time and feasible methods
- 04
Recalculate all paths after every compression increment
- 05
Handle multiple critical paths with shared or paired actions
- 06
Compare added direct cost with time-related project benefit
Worked example · free
Choose the cheapest effective two-day reduction
- +1Crash B by one day for 900 dollars because A-B-D is initially the only critical path.
- +1Both paths now take eleven days and both are critical.
- +1For the next day, compare shared A at 1,200, shared D at 1,600, or B plus C at 1,400.
- +1Crash shared A by one day because it shortens both critical paths at the lowest effective cost.
- +1The project reaches ten days for an added direct cost of 2,100 dollars.
Key terms
- Project compression
- The deliberate reduction of project duration by shortening effective activity durations or changing sequence and method.
- Crash time
- The shortest feasible duration for an activity under the defined accelerated method.
- Normal time
- The activity duration associated with its normal method, resource arrangement and direct cost.
- Crash slope
- The increase in direct cost divided by the reduction in activity duration between normal and crash points.
- Time-related cost
- Project cost that changes with overall duration, such as site overhead or another completion-linked consequence.
Project Compression and Cost Trade-offs FAQ
Why not crash the activity with the lowest cost slope?
A low slope saves project time only when the activity lies on every path that must shorten in that increment. Crashing non-critical work can increase float without changing completion, producing cost with no project-time benefit.
What happens when a second critical path appears?
Further reduction must shorten both paths. A shared crashable activity may do that once, or separate activities on each path must be shortened together. Add their marginal costs before choosing the action.
Does adding workers always reduce duration?
No. Workface limits, congestion, supervision and rework can prevent proportional improvement. Define the accelerated method and attainable production rate, then test resources and safety before accepting a reduced duration.
When should compression stop?
Stop when the target is reached, no effective option remains or the next marginal reduction costs more than its time-related benefit. Recalculate all critical paths first.
Exam move
Practise compression as an iterative table. List every path length, identify all critical paths, list crashable activities on those paths and calculate their marginal cost. Buy only one time increment, update activity duration and recalculate the network. Continue until the target is reached, no further effective reduction is feasible or the marginal cost exceeds the benefit.
Include examples where a cheap non-critical activity is a distractor and where a second critical path appears after the first crash. For multiple paths, compare a common activity against the combined cost of separate reductions. Finish every problem with duration saved, added direct cost, the new critical paths and the operational assumptions behind the crash method.
Then compare the cost with avoided overhead, earlier benefit or delay consequence. The shortest programme is not automatically the most economic or reliable programme. Prepare a crash table containing normal duration, crash duration, normal cost, crash cost, maximum reducible time and cost slope for every candidate. Exclude an activity only after explaining why its method cannot be shortened.
At each iteration, highlight all current critical paths and identify the cheapest effective set that reduces every controlling branch. If one shared activity lies on all paths, compare its slope with the combined slopes needed on separate branches. Reduce by the smallest increment that reaches a duration bound, creates another critical path or meets the target, then redraw.
Keep a cumulative direct-cost column and a separate estimate of time-related overhead or earlier-use benefit. The economic finish occurs where another day of acceleration costs more than it saves, subject to milestone and risk constraints. Challenge linear crash slopes when overtime fatigue, congestion or premium procurement makes marginal cost rise.
Record whether a proposed method changes quality, safety, logic or resource availability elsewhere. Also test whether the nominal saving survives calendars and approval dates. End with four reported values: achieved project duration, total time saved, added direct cost and remaining critical paths.
A recommendation should state the operational method and its assumptions, because choosing a cell in a cost table is not the same as proving the shorter programme can be delivered.
