To identify and minimize errors, incompleteness, wrong classification, and gaps in the information obtained from the target respondents.
Respondents often fail to answer a single question or a section of the questionnaire, either deliberately or inadvertently
A response might be unclear which of the two boxes is checked in a multiple response system
Sometimes two responses can be logically inconsistent
In a long questionnaire with hundreds of attitude or image questions, a respondent might rebel and check the same response (on a Likert scale, for example) for a long list of questions
An inappropriate respondent may be included in the sample