The Art of Statistics: Learning from Data

by David Spiegelhalter

Many people have a vague idea of deduction, thanks to Sherlock Holmes using deductive reasoning when he coolly announces that a suspect must have committed a crime. In real life deduction is the process of using the rules of cold logic to work from general premises to particular conclusions. If the law of the country is that cars should drive on the right, then we can deduce that on any particular occasion it is best to drive on the right. But induction works the other way, in taking particular instances and trying to work out general conclusions. For example, suppose we don’t know the customs in a community about kissing female friends on the cheek, and we have to try to work it out by observing whether people kiss once, twice, three times, or not at all. The crucial distinction is that deduction is logically certain, whereas induction is generally uncertain.

Deduction is certain while induction is generally uncertain