What is non-response bias?

A nonresponse bias a form of statistical bias that occurs when the answers of respondents vary from the answers of those who did not respond. The unknown answers from the nonrespondents can have an effect on the results.

This type of bias often occurs in questionnaires and surveys and can take two primary forms. The first form is item nonresponse, which refers to respondents agreeing to take part in a survey but not answering specific questions. The second form is referred to as unit nonresponse, in which an individual designated to take part in a survey isn't available to participate.