Base rates first

When judging a single case — a project, a startup, a plan — it is natural to start from its details. The details are vivid and specific. But the most useful number is often a boring one: how often do things of this kind work out in general?

How to use it

Before looking at the specifics, find a reference class and its typical outcome. How long do similar projects usually take? What share of similar attempts succeed? Start from that number, then adjust for what is genuinely different about this case.

Where it misleads

Choosing the reference class is itself a judgement call, and a poorly chosen class gives a confident but wrong anchor. Try two or three classes and see how much they disagree.