Sample

Because measuring an entire population is often impossible, you gather data from a smaller sample and use it to make estimates. For example, polling 1,000 voters to estimate how all voters will act treats those 1,000 people as the sample. A numerical summary of a sample, such as the sample mean (written $\bar{x}$, read as x-bar), is called a statistic. A well-chosen random sample lets that statistic stand in for the unknown population value.