Principle
Let ranges narrow only when uncertainty is actually removed
Time passed is not evidence gained.
When it fits
- A project gets older, so forecasts become narrower by convention even though key unknowns remain.
When to avoid it
- A wide range can be unhelpful if it never supports a decision; pair uncertainty with the next information that can reduce it.
Why it matters
At each stage, identify which uncertainties were resolved and which remain. Narrow the range only when scope, design, dependencies or throughput have become materially better known. If new uncertainty appears, the honest range can widen again.
An example
After a prototype validates the interface and throughput, the range narrows; after an unexpected compliance review appears, it widens again.
Check your result
The width change has an evidence explanation.
Keep this limit in mind
- A wide range can be unhelpful if it never supports a decision; pair uncertainty with the next information that can reduce it.
Evidence and sources
Supports
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