Denominator Neglect

Picture two urns stood on a table in front of you.

You’re given the opportunity to pick a marble from one of them, and drawing a red marble wins a prize.

The first urn has 10 marbles in it, 1 of which is red.

The second urn has 100 marbles in it, 8 of which are red.

Which urn would you choose? It doesn’t seem a tricky decision: your chances of drawing a red marble out of the first urn are greater (10%) than your chances of drawing a red marble out of the second urn (8%).

And yet, as Daniel Kahneman describes in Thinking, Fast and Slow:

‘About 30-40% of students (the survey participants) choose the urn with the larger number of winning marbles, rather than the urn that provides a better chance of winning.’

… Read more: Attest – Official Publication

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