Third mailings in epidemiological studies: Are they really necessary?

Abstract

Introduction. Whether or not third mailings are appropriate or worthwhile in postal epidemiological studies has not been thoroughly investigated and requires examination. Methods. A self-completion postal questionnaire of 2184 individuals was conducted in 2000. The socio-demographic and health characteristics of four groups of individuals (first mailing respondents, second mailing respondents, third mailing respondents and non-respondents) were compared. Results. Some significant differences between the groups were found, however, the inclusion of respondents to the third mailing did not significantly change the overall characteristics of respondents compared to non-respondents. Discussion. When differences do exist between respondents and non-respondents, our results suggest that a third mailing is unlikely to remove many of these differences. The study supports our preivous suggestion that the effort and resources expended in carrying out a third mailing may not be justified.