Studies of interventions to increase response rates on self-administered, postal questionnaires have manipulated a variety of variables including elements of questionnaire design and presentation, sponsorship, pre-notification, postage, envelopes, incentives, rewards, and follow-up techniques. The systematic review by Edwards et al. [1] included 292 trials including 258,315 participants and 75 different strategies for increasing response to postal questionnaires. Various types of postage, on both out-going and return envelopes affected response rate. A search of the literature by Edwards et al. did not uncover any international postal questionnaire studies in which the respondents were provided with return postage in some form.

We wanted to see if providing return postage to international respondents had an effect on response rate. International Postage Vouchers – "Coupon-Réponse International", which can be exchanged at the respondents' post office for stamps, was the only form of international return postage available in Norway. Although this is a cumbersome method of providing return postage, we wanted to see if eliminating postal costs that might be a barrier for some respondents would have an effect. In addition, we hypothesized that the gesture of providing the vouchers would serve as an incentive and it would be up to the individual how to use the vouchers, either for returning the questionnaire or to redeem stamps for personal use.

Of the 97 questionnaires assumed to have reached the addressees, 12.4% (n = 12), 95% C.I. (5.82%, 18.92%) were returned filled in. Of these, one questionnaire from the experimental group was returned using United Parcel Service, a private postal service, and is included in the analysis as a response in the experimental group. Otherwise, it was not possible to see which respondents in the experimental group had or had not made use of the vouchers, or if stamps had been procured by other means (had on hand, affixed by employer, etc.) Two questionnaires sent to respondents in the control group were filled in and returned by eligible colleagues of the addressees and are included in the analysis.

There was a higher response rate in the control group (16%, n = 8) than in the experimental group (8.2%, n = 4), but there was no difference in the response rate estimates for the two groups in the population (Z = 1.26, p = 0.23, and 95% C.I. -4%, 21.5%).

Many journalists who were sent the questionnaire might have excluded themselves from the study because they did not write about health matters, resulting in an overall low response rate. Nevertheless, because of random allocation it is assumed that underlying or confounding variables would have been equally distributed in the control group and the experimental group.

As a matter of interest, there was a significant difference (p = 0.001) between the response rate for the e-mail contacted, Internet-based cohort (32%) [2] and the postal cohort (12.4%), resulting in a relative risk = 2.6. This could quite possibly be due to a selection bias between the two groups. Our result goes in the opposite direction from that of VanDenKerkhof et als study, where e-mail contacted, Internet participants were only half as likely as postal-contacted participants to respond to the questionnaire (35% vs 69%, relative risk = 0.51) [3].

The most optimistic interpretation of the 95% CI (-4, 21.5%) is that with sending vouchers, for every 25 vouchers sent the number of responses increases by one compared to not sending vouchers.

We are in danger of committing a Type II error as the study is underpowered (post-hoc calculation = 0.13) However, the results of this study are consistent with the findings of Groves and Olsson in the US that the provision of a stamped, return-addressed envelop compared with the provision of only a return-addressed label did not make a difference in the response rate [4]. When these two studies are considered together, it appears that the provision of return postage does not increase questionnaire response rates from the workplace.