Avoid assigning prior probability 0 or 1 unless logically certain.
Lindley named it for Cromwell's 1650 plea to consider that you may be mistaken. Formally, a prior of zero cannot be updated by any evidence whatever — certainty is unrevisable by construction.
In practice
- An investigator who has ruled out one suspect entirely cannot revise, whatever the new evidence shows.
- A forecaster who says a project has "zero chance" of slipping has left themselves nowhere to go when the first milestone moves.
- Further reading
- Wikipedia
- Subject
- Decision-making & Reasoning
- Essay
- Razors, rules and the tools of careful thought