6.4.1—Samples and populations

A population contains all units of interest; a sample is selected to estimate population characteristics. A parameter describes the population, while a statistic is calculated from the sample.

Define the target population before sampling and consider coverage, non-response and selection bias. Larger samples reduce random error but do not automatically remove systematic bias.

A survey of 500 randomly chosen voters is a sample; the proportion supporting a policy in all eligible voters is a population parameter.

A sample statistic is not the parameter itself, and a large biased sample can still mislead.