Differences between dental health data obtained by interviews and questionnaires

Two hundred and eighty-six persons out of a population of 358 were interviewed about dental health. Approximately 9 months later the same group was posed identical questions in a self-administered questionnaire. An open-ended question about reasons for loss of teeth showed nearly 50% more answers on the questionnaire than in the interview. The number of answers in the questionnaire fell with increasing age and decreasing income and/or lower social class, while sex, age, income, and social class seemed to have only a minor effect on the number of answers in the interview. The number of two-reply categories varied between the interview and the questionnaire. To questions about water fluoridation and covering of dental expenses by Social Security, married couples gave more often identical replies on the questionnaire than in the interview. The findings imply that the method itself, i.e. the choice between the interview and questionnaire methods, is of importance for the results as such.