What usually happens in comparable situations?
A forecast needs more than the details of one appealing story. If similar projects usually take six weeks, a new estimate of two weeks needs a clear explanation. A base rate provides a starting point from comparable cases, which can then be adjusted using evidence specific to the current situation.
Go deeper
Start outside the individual story
A project team lists its planned tasks and predicts a two-week delivery. The estimate sounds convincing because every step appears manageable on its own.
Now suppose five genuinely comparable projects took between five and seven weeks. That history is relevant even if the new team feels confident.
A base rate describes what happens across an appropriate set of cases. It gives a forecast an external starting point.
The comparison group matters. A small revision should not automatically inherit the timeline of a complete rebuild. Once the group is chosen carefully, ask what concrete differences justify moving away from its usual result. New tools, fewer requirements or extra staff may matter, but enthusiasm alone does not explain the gap.