Heuristic
Use a range when the uncertainty is asymmetric
A symmetric plus-or-minus can hide a long tail.
When it fits
- The most likely duration is known but delays can be much larger than early finishes.
When to avoid it
- Three points do not establish exact probabilities; avoid fake precision unless you have a defensible distribution.
Why it matters
When the downside and upside are not balanced, record reasonably optimistic, most-likely and reasonably pessimistic values rather than a central estimate with a cosmetic symmetric margin. State what assumption changes between the points.
An example
A data cleanup may take 1 day if errors are already classified, 2 days most likely, and 5 days if manual reconciliation is required.
Check your result
The range explains why the high side is wider or narrower than the low side.
Keep this limit in mind
- Three points do not establish exact probabilities; avoid fake precision unless you have a defensible distribution.
Connected ideas
Useful withSweep one uncertain assumption across a plausible range
Evidence and sources
Supports
IPA cost-estimating guidance recommends documenting assumptions and representing uncertainty with reasonably optimistic, most-likely and reasonably pessimistic positions where appropriate.
Three-point estimates are inputs to judgment or modelling, not proof of a probability distribution.
Cost Estimating Guidance · Estimate ranges and assumptions