State your prior probability before seeing the evidence

Before looking at any data, commit to a numerical estimate of how likely something is.

Why it works

Naming a prior before seeing evidence does two things: it makes the subsequent update visible (you can see how far you moved), and it forces you to confront base rates — the background frequency of an event — rather than reasoning only from the case at hand. Research on base rate neglect shows that people systematically ignore background frequencies when case-specific information is available; a stated prior anchors them.

How to do it

1. Before examining any new evidence, ask: "What is the base rate for this type of thing?"
2. State a number: "I think there is roughly a 20% chance this is true before I look at any evidence."
3. Write it down — a mental estimate evaporates under the pressure of incoming evidence.
4. Only then look at the evidence, so you can measure how much it actually moves you.

Evidence

Base rate neglect is one of the most replicated findings in judgment research — people reliably underweight background frequencies when presented with individuating case information. Bar-Hillel’s work isolated when the effect is strongest: individuating information dominates precisely when people judge it relevant to the case, even when the base rate is the more diagnostic input. (rct)

The prior-setting habit as a practical countermeasure is widely endorsed in rationality training but has fewer direct experimental tests than the bias it targets.

Sources

• Kahneman & Tversky (1973), on the psychology of prediction, Psychological Review
• Kahneman, D., & Tversky, A. (1973). On the psychology of prediction. Psychological Review, 80(4), 237-251.
• Bar-Hillel, M. (1980). The base-rate fallacy in probability judgments. Acta Psychologica, 44(3), 211-233.

Common mistake

Setting a "prior" after glancing at the evidence, then believing you are reasoning from base rates when you are actually reasoning from the evidence twice.