Abstract
BackgroundIn research with long-term follow-up and repeated measurements, quick and complete response to questionnaires helps ensure a study’s validity, precision and efficiency. Evidence on the effect of non-monetary incentives on response rates in observational longitudinal research is scarce.ObjectivesTo study the impact of two strategies to enhance completeness and efficiency in observational cohort studies with follow-up durations of around 2 years.Method and interventionIn a factorial design, 771 children between 2 and 5 years old and their parents participating in a prospective cohort study were randomized to three intervention groups and a control group. Three types of lotteries were run: (i) daytrip tickets for the whole family to a popular amusement park if they returned all postal questionnaires, (ii) €12.50-worth gift vouchers for sending back the questionnaire on time after each questionnaire round and (iii) a combination of (i) and (ii).Main outcome measuresPrimary outcome was the proportion of participants who returned all questionnaires without any reminder. Secondary outcomes were ‘100% returned with or without reminder’, ‘probability of 100% non-response’, ‘probability of withdrawal’, ‘proportion of returned questionnaires’ and ‘overall number of reminders sent’.Statistical analysisAfter testing for interaction between the two lottery interventions, the two trials were analysed separately. We calculated risk differences (RD) and numbers needed to “treat” and their 95% confidence intervals.ResultsDaytrip nor voucher intervention had an effect on the proportion of participants who returned all questionnaires (RD −0.01; 95% CI-0.07 – 0.06) and (RD 0.02; 95% CI-0.50 – 0.08), respectively. No effects were found on the secondary outcomes.ConclusionOur findings do not support the idea that lottery-style incentives lead to more complete response to postal questionnaires in observational cohort studies with repeated data collection and follow-up durations of around 2 years.
Highlights
In research with long-term follow-up and repeated measurements, quick and complete response to questionnaires helps ensure a study’s validity, precision and efficiency
Our findings do not support the idea that lottery-style incentives lead to more complete response to postal questionnaires in observational cohort studies with repeated data collection and follow-up durations of around 2 years
The trial we present here was nested in the ARCADE prospective cohort study in young children at high risk of developing asthma
Summary
In research with long-term follow-up and repeated measurements, quick and complete response to questionnaires helps ensure a study’s validity, precision and efficiency. Evidence on the effect of non-monetary incentives on response rates in observational longitudinal research is scarce. One of the four RCTs evaluated the effect of a monetary (cash or check), one the effect of a non-monetary incentive (pencil) on retention rates of postal questionnaires [2,3]. Follow-up questionnaires were sent after 8 to 10 years and the monetary incentives were a USD 1 bill, a USD 2 bill or a check worth USD 5. White et al included a pencil in a follow-up mailing sent 2 years after men and women completed a baseline cohort study questionnaire [3]. We were especially interested in the effects of somewhat larger and seemingly attractive incentives in which a lottery determined who won
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