Border police looking for illegal drugs with a specially trained dog: If they check every tenth car, they are taking an unbiased sample.

In statistics, a sample is part of a population. The sample is carefully chosen. It should represent the whole population fairly, without bias.

When treated as a data set, a sample is often represented by capital letters such as \displaystyle{ X } and \displaystyle{ Y }, with its elements being represented in lowercase (e.g., \displaystyle{ x_3 }), and the sample size being represented by the letter \displaystyle{ n }.