Protocol
Pin the scope snapshot to every material forecast
A forecast can be wrong; it can also be about a different job.
When it fits
- A forecast is later judged against a project that changed shape.
When to avoid it
- Do not freeze harmless wording changes into formal re-estimation; use a materiality threshold appropriate to the work.
Why it matters
Store the estimate with its scope version or a short snapshot of included deliverables, assumptions and dependencies. When scope changes materially, create a new forecast and preserve the old one for learning rather than silently editing history.
Steps
- Forecast date recorded.
- Scope/version recorded.
- Key assumptions recorded.
- Material change creates a new forecast.
- Old forecast preserved for calibration.
An example
The original estimate covered 1,000 records; after the scope grows to 1,800, the revised estimate is a new forecast rather than a rewrite of the original.
Check your result
Forecast error can be separated from scope change.
Keep this limit in mind
- Do not freeze harmless wording changes into formal re-estimation; use a materiality threshold appropriate to the work.
Connected ideas
Use beforeRe-estimate when evidence changes, not when anxiety spikes
Evidence and sources
The 2026 Green Book recommends explicitly accounting for optimism bias in cost, benefit and duration estimates and using historical forecast errors from similar proposals where available.
The guidance is for UK public appraisal; the transferable principle is empirical correction, not a universal percentage uplift.
The Green Book (2026) · Optimism bias
IPA guidance notes that uncertainty and estimate range depend on the maturity and variability of input data, so estimates should evolve as design and evidence mature.
A later estimate can still be wrong; maturity narrows uncertainty only when real information has improved.
Cost Estimating Guidance · Estimate maturity and range