Can Follow-up Response Rates to a Mail Survey Be Increased by Including Another Copy of the Questionnaire?

In a mail survey of business respondents, this experiment tested the hypothesis that return rates from follow-ups which included another copy of the questionnaire would be more effective than letter-only follow-ups. Sending another copy of the questionnaire in the follow-up could increase returns for a number of reasons, such as providing another questionnaire for respondents who had discarded the original. The two methods were equally effective in the first follow-up, however, in the second follow-up returns from the letter-plus-questionnaire were almost twice that of the letter-only group. The results suggest that in planning a mail survey, it may be appropriate to consider a letter-only first follow-up and a letter-plus-questionnaire second follow-up.