Estimates are guesses.
Estimates miss for predictable reasons. Knowing them won't make you psychic, but it will make you less surprised.
01
Planning fallacy
We picture the best case. Kahneman and Tversky named it in 1979.
02
Unknown unknowns
The problems you can't see until you start.
03
Anchoring
The first number said out loud sticks.
04
Negotiated down
Estimates bargained to fit the date someone wants.
05
Hidden work
Reviews, testing, deploys and docs left out.
06
Interruptions
Support, meetings and leave eating capacity.
Source: Society for Personality and Social Psychology: The Planning Fallacy: An Inside View ↗
Estimates are guesses. Treat them like it.