Term Definition
A sample element is selected, but an interview does not take place (for example, due to noncontact, refusal, or ineligibility).

Sampling units that were potentially eligible but could not be reached.

Synonyms - noncontacts, non-contact, non-contacts
The failure to obtain measurement on sampled units or items. See unit nonresponse and item nonresponse.
Nonresponse bias
The systematic difference between the expected value (over all conceptual trials) of a statistic and the target population value due to differences between respondents and nonrespondents on that statistic of interest.
Nonresponse error,

Survey error (variance and bias) that is introduced when not all sample members participate in the survey (unit nonresponse) or not all survey items are answered (item nonreponse) by a sample element.

Synonyms - nonresponse errors, nonresponse error
Nonresponse follow-up
A supplemental survey of sampled survey nonrespondents. Nonresponse follow-up surveys are designed to assess whether respondent data are biased due to differences between survey respondents and nonrespondents.