Principle
Estimate active effort and elapsed time separately
Two hours of work can still occupy three calendar days.
When it fits
- A task contains approvals, queues or external waits and one duration number hides them.
When to avoid it
- Wait times can dominate delivery and may be highly variable; do not pretend they are fixed just because the active work is well known.
Why it matters
Estimate hands-on effort separately from elapsed time. Then list the waits, calendars and dependencies that turn effort into delivery time. Use effort for capacity planning and elapsed time for stakeholder commitments; do not substitute one for the other.
An example
A 90-minute data check needs a two-day elapsed window because the source owner may answer only next morning.
Check your result
The forecast can explain why elapsed time differs from work effort.
Keep this limit in mind
- Wait times can dominate delivery and may be highly variable; do not pretend they are fixed just because the active work is well known.
Connected ideas
Useful withPut the slow external dependency on the forecast
Evidence and sources
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
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