Base rates

Start with how often it happens

A base rate is how often an outcome occurs across the whole reference class, before you know anything about the specific case in front of you.

Reasoning from base rates means estimating by:

  1. Naming the reference class the case actually belongs to.
  1. Starting from that class's historical frequency rather than the details of the case.
  1. Adjusting for the specifics only as far as they are genuinely diagnostic.

The vivid particulars of a case rarely justify abandoning what usually happens.