I’ve long been interested in the phenomenon of statistical literacy – that is, the ability understand (or misunderstand) statistical information, especially information dealing with probability.

This USA Today article illustrates this phenomenon:

When your local weather forecaster announces that there is a 30% chance of rain tomorrow, not everyone knows what that means.

Some think it means 30% of an area will get rain. Others think it will rain for 30% of the day. In fact, of all the forecast terms used by meteorologists, this remains one of the most baffling to the public.

Some people don’t understand that the forecaster simply means there’s a 30% probability it will rain at some point during the day. Susan Joslyn, a senior lecturer in the psychology department at theUniversity of Washington in Seattle, and colleagues have been studying such confusion.

More information on statisical literacy can be found here.

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Chance statements emphasize the chance of an uncertain outcome (30% chance of [some] rain) but often fail to state anything about the process (some rain anywhere in a given geographic area on a given day). A clearer statement is to use percent grammar to express the relative frequency:

“Of the past days like tomorrow, 30% had rain somewhere in the area in question.”

Unless the rain covers the entire geographic area in question, the chance that a randomly selected individual will experience rain tomorrow is generally less than the average for the entire area.