🚕Base rate

How often something happens in the relevant group before you look at the case in front of you. The starting frequency, ahead of any particular evidence.

Ignoring it is the oldest mistake in 🔬 Judgment under Uncertainty - Heuristics and Biases: a vivid description beats a dull percentage, and the percentage was the better guide. Taught in 🎓 Lecture 3 - Probability and base rates, in the worst week of my February. The taxi example in 📱 Base rates in sixty seconds is the one that stuck.

Origin: base comes through Latin from Greek basis, a step or pedestal, the thing something stands on. Rate is from medieval Latin rata, as in pro rata parte, a reckoned share.

Filed under 02. 🎲 Decisions under Uncertainty.

An example from my work. A new flag lights up: late at gate. It looks urgent. The question that comes first is how often exceptions of this type resolve themselves with nobody touching them.

If, say, nine in ten clear within the hour, then the flag on its own tells a dispatcher very little, and ranking it at the top teaches them to ignore the top. I had the alarm and no denominator. That is 💎 A number without its base rate is a story.