Validity of a questionnaire survey: the role of non-response and incorrectanswers

Errors in questionnaire surveys are usually of one of two sources: non-resp onses or incorrect answers. The aim was to investigate the validity of a qu estionnaire survey and to estimate the respective bias of these answers. Of 9,283 subjects selected to receive a questionnaire by post, 3,949 (43%) re sponded, and, of these, 3,400 correctly reported their Swedish social secur ity number. Answers in the questionnaire survey were given as proportions o f the claims registered at local insurance offices. In the group of respond ents who had correctly reported their social security number, the answers w ere compared individually with the registrations in dental insurance claims . In Sweden, these claims are labeled with the patient's social security nu mber and it is thereby possible to make such comparisons. It was shown that errors were caused by non-response and also by respondents giving incorrec t answers. Incorrect answers accounted for approximately one-third of the t otal bias. The remaining bias was caused by a non-response error. It is con cluded that questionnaire studies have a bias caused by both non-response a nd incorrect answers and that together these can be substantial. Scientific reports that include questionnaire surveys must describe the procedure car efully. If possible, other sources of information should be considered.