Extreme project schedule over run

There is often a sense that project schedule risk distributions fail to exhibit the long right-hand tails that experience suggests should show up in a realistic assessment of schedule risk. This extended tutorial demonstrates quantitatively that unexpected extensions of time might arise not from unforeseen risks or understated risks, nor from a bias towards optimism or over-confidence, but through a systematic interaction between the risks that are taken into account by conventional schedule risk modelling and firefighting behaviour stimulated by schedule slippage.

The mechanism described here shows how growing delays can cascade through a project when:

  • Anxiety about poor performance leads to the adoption of inefficient practices, and
  • Planning assumptions are undermined as task sequencing and resource timetabling are disrupted.

A simple interaction between straightforward risks affecting a schedule, duration uncertainty, and a degradation of productivity can even generate bimodal distributions of schedule outcomes (Figure 1). Large overruns may arise under modest assumptions about when and by how much productivity will degrade as a schedule comes under pressure. As a project becomes increasingly sensitive to pressure – increasingly fragile – its forecast duration distribution can be changed dramatically.

Read the full tutorial here.

Figure 1: Effects of increasing fragility