An increase matters, not the actual value: early bird incentives in longitudinal surveys

This paper provides unique new evidence regarding the effect on response rates of increasing the value of an early bird incentive (EBI) offered to sample members completing an online questionnaire during the first five weeks of fieldwork. The experiment analysed in this paper, embedded in wave 12 of Understanding Society, a longitudinal mixed-mode survey, tested an increase of the EBI from £10 to £20. Moreover, the experiment additionally covered a subsample who were being administered the web-first mixed-mode design for the first time, having previously been administered a CAPI-only design and, therefore, had not been offered the EBI. This allowed us to disentangle the effect on response rates of the higher value of the incentive from the effect of raising the incentive value. We also examined the effect of the increased incentives on fieldwork efforts and sample composition. We found that increasing the value of the incentive had a positive effect on response rates for panel members who had been offered the EBI previously, whereas the higher value had no significant effect on those who had not previously been offered an EBI. The positive effect of the EBI on response was particularly pronounced for certain low-response propensity groups.