For each wave of a longitudinal survey, non-response may occur in one of two basic forms: wave nonresponse where the respondent answers no questions for the entire wave interview and item nonresponse where the respondent answers some questions but not all.

Wave nonresponse for a longitudinal survey is different from unit nonresponse for a cross-sectional survey. For the latter, typically the only variables available are those from sample frame data and interviewer notes. But because a longitudinal survey is conducted over time, the values of some variables, such as demographic variables, are available from earlier waves and are not expected to change much. These presumed known values can be used to contribute to the resolution of missing data values. Even more important, for a missing variable one may have access to values of this variable and other correlated variables as reported in other waves (longitudinal patterns). In some cases, these two factors can contribute to a reasonable resolution of missing values for wave nonresponse, and this sort of option is not usually available for cross-sectional surveys.