base rate neglect

Base rate neglect or base rate fallacy is a common cognitive bias, where one ignores the base rate (how common something is), when making a judgment.

For example, imagine a test for a rare form of cancer that is 100% accurate in detecting if a person has the cancer and 99% accurate when they don't. You find you have tested positive. At first this seems very worrying; however, if the base rate is that only 1 in a million people have the cancer but the other 999,999 don't, then for every person who has the cancer and tests positive there are 1% of 999,999, that is around 10,000, who test positive and are in the clear. It may be worth having further tests, but you are no more likely to have the disease than to be killed on the road that year.