As in other types of surveys, we can distinguish between unit nonresponse, where the expected (invited)  eligible units do not participate in the survey, and item nonresponse, where the units participate, but certain responses are missing to some questions.

In addition, due to the availability of paradata – i.e., data from the process of answering a Web survey questionnaire that are stored in the log files together with the responses (Couper 2005) – a variety of additional nonresponse measures and patterns may be detected in Web surveys (Bosnjak 2000). In particular partial nonresponse, where respondents answer only part of the questionnaire and then drop-out, is an issue requiring further discussion.

Here we predominantly focus on unit and partial nonresponse, while leaving the item nonresponse aside because it is not so much problem of participation in Web surveys, but is more related to Web questionnaire design issues, which is not our focus here.