A randomised controlled trial to assess the effectiveness of offering study results as an incentive to increase response rates to postal questionnaires [ISRCTN26118436]

1038 women aged over 70 years were remotely randomised by computer in a 3:1 ratio. 250 participants did not receive the offer of knowing the results of the trial and 788 participants were offered the results of the trial in a postal questionnaire. The main outcome measure was response rate. Chi square test was used to evaluate the overall differences in response rate between the two groups. An adjusted analysis, adjusting for whether the participant was taking calcium and age was also undertaken.The response rates were not significantly different Odds Ratio 0.88 (95% confidence intervals 0.48 to 1.63) p = 0.69.Offering study results to women living in the community aged over 70 does not increase response rates to postal questionnaires. Although researchers have an ethical obligation to offer participants study results, since 10% of women did not wish to receive the results, investigators should give participants the option to opt out of receiving the study's results.Postal questionnaires are widely used to collect data on participants in health research. They are an attractive means of collecting data, because they are easy to administer and may be the only economically viable method of collecting data on large numbers of participants who may be geographically dispersed. However, if there is a poor response rate to these questionnaires the validity of the study may be at risk for two main reasons. First, the statistical power of the study will be reduced. Second, bias may be introduced if non-responders differ significantly from those who do respond. In other words low response can threaten the internal validity of randomised trials as bias can be introduced and the external validity of surveys are undermined as non-response will prevent random sampling. Identifying effective strategies to increase response rates to postal questionnaires is therefore highly relevant to researchers since it could improve the quality of their research.Offering participants the chance