I recently watched a competitive swimming heat on television. The race sticks in my mind because 50% of the athletes, four in total, were from the United States. Knowing very little about each participant, nor who the favourite was for the race, at first glance, it looked like the United States had to place in the top three. The result: none of them finished in the top three.

In this case, the size of each category, the country each athlete represented, incorrectly led me to believe they had a better chance of making it into the top three. I incorrectly believed “they had the numbers on their side”.

The category size bias demonstrates how our probability judgments are often inaccurate. Category size can impact the perceived likelihood of a specific outcome, such that an outcome classified into a large (vs small) category is perceived as more likely to occur.