Non-response error happens when selected respondents do not participate or cannot be contacted, possibly creating bias if non-respondents differ systematically from respondents.

For example, if you send out an internet survey to estimate the percentage of the world population has internet access, then anyone who would respond "no" will not be able to access your survey.

Similarly, if you ask students to share their grade on the latest math test, those who are unhappy with their scores are less likely to respond, biasing your sample.