# Category Size Bias

## Why “Red 7” is just as likely as “Green 0”

Category Size Bias: We believe items in larger categories to be more likely to occur than ones in smaller categories.

I don’t visit casinos very often, but when I do Roulette is one of the few games I like to gamble on. For those unfamiliar, a Roulette wheel has 18 black slots, 18 red slots and 2 green slots.

The probability of landing on any of the slots is exactly the same. However, many of us make the mistake of thinking that the ball is more likely to land on the black or red numbers than the green numbers. This is because there are more black and red slots (18 each) than green slots (2). Imagine a Roulette wheel with no colours and try to guess where the ball is most likely to land. It’s impossible. The probability of landing on all slots is identical.

When standing in front of the Roulette table our rational judgment of the probability of landing on a ‘red 7’, for instance, is affected by the category it sits within. Red’s a bigger category than green, so we assume it to be the more probable of the two.

We mistakenly believe items in larger categories to be more likely to occur than ones in smaller categories.

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