The sampling frame is the practical list or system used to reach possible participants, such as residential addresses, telephone numbers, or a voter file. A frame can omit part of the population. The selected sample can shrink again when some people cannot be contacted or refuse. A careful report lets you trace the path from target population to frame, selected cases, completed interviews, and weighted estimate.

A population value is often called a parameter, while the value calculated from a sample is a statistic. Researchers use the statistic to estimate the parameter. That inference is credible only when selection, coverage, response, and adjustment support it. A sample does not need to include everyone, but it must give the intended population a fair and explainable route into the evidence.

Video lesson: Science Behind the News: Opinion Polls & Random Sampling