Chocolate frogs do not increase completion of parent survey: Randomised study

Four months into a year-long, national survey assessing parents' experiences of a child's diagnosis of autism spectrum disorder, our response fraction was only 23%. We aimed to determine whether including a chocolate incentive in the postal survey would increase the response fraction. Families enrolled between 15 March and 25 May 2012 were randomised to receive a chocolate frog versus no chocolate frog. Both groups received a written reminder and replacement survey 2 weeks after the survey was posted and up to two telephone reminders thereafter. We analysed the effect of the incentive using χ2 tests for the categorical response variable and t-tests for the continuous reminder and length of response variables at the end of (i) randomisation and (ii) the study (1 November 2012). A total of 137 families were randomised in the 6-week period. Parents who received an incentive were more likely to return a completed survey in the 6 weeks than those who did not (21% vs. 6%, P = 0.009). This effect faded by the end of the study (53% vs. 42%, P = 0.4). There were no differences between groups at either follow-up in the number of reminders that parents received or the number of days it took parents to return the survey. Including a chocolate-based incentive does not significantly increase response rate in a postal survey over and above standard reminder techniques like posting follow-up survey packs or phoning families.