The marbles in each urn will be thoroughly mixed, and you, blindfolded, may reach in and take one marble from the urn of your choice. If you select a red marble, you win $100. Which urn will you choose?
Urn A contains 10 marbles, one of which is red. Urn B contains 100 marbles, 8 of which are red.
Make a selection above before reading further.
Some people may find the question too easy, a matter of simple math. However, when this experiment is conducted, between 30 and 40 percent of participants choose the urn that will not give them the best odds. It should be easy to calculate that you have a 10 percent chance of success with Urn A and an 8 percent chance with Urn B, so Urn A is the best choice. But many people choose Urn B, thinking only of the higher number of red marbles. This is the cognitive bias of denominator neglect: 8 is greater than 1, but 1/10 is greater than 8/100.