Thirty per cent of what?

A forecast says "30% chance of rain tomorrow". A study that asked people in five cities what that means found at least three answers: rain for 30% of the day, rain over 30% of the area, or rain on about 3 of every 10 days like tomorrow. Only in New York did most people give the forecaster’s meaning, the third one.

The trouble is not arithmetic. A probability is a share of something, and the forecast does not say of what. The fix the researchers suggested is to name the reference class: "on days like tomorrow, it rains here on about 3 in 10".

Teenagers on Spark & Anvil meet the same question in 30% Chance of Rain, and younger children count rainy days in a week, which is where the idea of "out of how many" starts.