The results from our study show that the use of economic incentives, such as small gifts (scratch cards) or a prize draw for gifts of a greater value (tablet computers) did not help increase the participation in a medical survey undertaken among people aged 55 – 64 years.
The participation rate was 54.5 %. This is lower than that reported from equivalent surveys in Norway previously (1, 6, 7, 27). Our results show that those who had declined to participate in the colonoscopy screening (those invited for screening) had a lower response rate than the control group. The proportion of those invited for screening who responded (43.5 %) was nevertheless considerably higher than those 11 % who responded in a similar study of non-attendees in a comparable survey on risk factors for colorectal cancer in 2006 (25). The higher rate of participation among non-attendees in our study was surprising, since the scope and topic of the study were fairly similar to the previous investigation. Possible explanations could be that our target group consisted of persons who were from five to ten years older than those in the previous study, that the questionnaire contained fewer questions and that we followed up the non-respondents with reminder calls.
The objective of surveys is to generate knowledge that is valid not only for those who have responded, but for a larger group of people. Therefore, it is desirable that the responding group be as «similar» as possible to the larger group, meaning that they constitute a representative sample. As a researcher one can control that those who are invited to participate in a survey are representative, by drawing a random sample. One cannot control, however, that those who actually respond to the survey are representative.
A high response rate may be an indication of a more representative sample, but there is no guarantee that this is the case. There may still be people with special characteristics who are less inclined to respond to the survey, and this may give rise to a measurement bias when compared to a situation in which all those included have responded. For example, people with low socioeconomic status and a high prevalence of lifestyle-related diseases are often underrepresented (1, 2), which results in artificially low prevalence figures for a number of diseases. Such bias is more discernible in measurements of the prevalence of a condition than in measurements of the association between exposure and an outcome (28).
In a Cochrane review on the use of incentives in surveys (not only medical ones), unconditional incentives that could be used by the participants irrespective of whether they participated or not (such as the scratch card in our study) had a greater effect than conditional incentives that were provided after the questionnaire had been completed (10). Our results showed no significant effect of the unconditional incentive (the ten-kroner scratch card). This tallies well with the results of another Norwegian study conducted in 2006, which found no increased response rate after distribution of scratch cards (22). Yet another Norwegian study from 2012 drew the opposite conclusion, however: enclosing a scratch card raised the response rate by 10 % compared to not giving out any prizes (21).
Feedback from those who undertook the reminder calls and telephone interviews indicates that many participants were highly motivated to respond. Many of them stated that colon cancer was a matter that concerned them and that they regarded as an important research topic. We thus have indications that the participants in our study initially had a strong internal motivation to respond. This may have contributed to the absence of any effect of the external economic incentives. Other factors that may help explain the incentives’ lack of effect include the fact that the study was undertaken in an age group that we may assume to be wealthier than young people; they can afford to buy their own scratch cards and tablet computers. Moreover, the questionnaire was relatively short, and we may surmise that this requires less external motivation than a long, time-consuming questionnaire.
More than 40 % of those invited for screening chose to respond to the questionnaire, despite having declined the colonoscopy screening examination. We expected to find a large co-variation between non-attendance of the screening and non-response to the survey. That this co-variation was lower than expected may indicate a difference in selection mechanisms. The reasons why someone failed to attend the colonoscopy screening may be health-related or of a practical nature, such as work duties or transport problems, but he or she may nevertheless have responded to the questionnaire which only required a few minutes to complete at home. In addition, there may have been a fear that the colonoscopy examination would be an uncomfortable experience, while this mechanism has no influence for the survey.
We found no increased response rate as a result of providing incentives to the participants in this medical survey – neither those incentives that were conditional upon participation, nor the unconditional incentives had any effect. In contrast, we observed a significant effect of reminder telephone calls to participants that had not responded and to whom we provided an opportunity to respond to the survey by telephone. This increase in response rate was especially pronounced among those who had been invited for screening, a group which is difficult to reach in such health studies.
Whether telephone reminder calls are more effective than sending reminders by mail, and whether alternative response modes alone increase participation, requires further study, as do any methodological, economic and ethical aspects of the various alternatives. When planning new studies within equivalent population groups, however, it may be appropriate to give priority to reminders and alternative response modes above provision of incentives.