Time Core · Contingency Planning · Module
Slippage
Where plans actually slip, by how much and why — established from history rather than from the assumption that this time will be different.
The idea
How it works
Estimates are optimistic in a consistent and measurable direction. The planning fallacy is one of the most robust findings in behavioural research and it applies to organisations that know about it.
The correction is not to try harder at estimating. It is to look at what happened to comparable past work and adjust by that ratio, which is uncomfortable and reliable.
Working with it
In practice
- 01
Keep the estimate and the actual
One line per project. Within a year the ratio is visible and is usually consistent.
- 02
Adjust by the historical ratio
Rather than by re-estimating more carefully. The second does not work and the first does.
- 03
Find the steps that always slip
Usually approvals, integrations and anything involving a third party. They are the same ones each time.
- 04
Separate slip from scope change
A project that took twice as long because it doubled in size did not slip. Mixing them corrupts the ratio.
One level in
The components of slippage
A component is something that exists afterwards which did not exist before — a deliverable or a mechanism, not an intention.
The correction for optimistic estimates is not estimating more carefully. It is adjusting by the historical ratio.
The other modules in contingency planning
Buffers
Reserve held deliberately and visibly rather than hidden inside individual estimates, where it is consumed without anyone noticing.
LearnThe fallback plan
What is done instead, prepared far enough in advance that it remains available when it is needed.
LearnDecision points
When the choice between continuing and changing course has to be taken — derived from the lead time of the alternatives rather than from the deadline.
Learn