Effects of a Time-Limited Push-to-Web Incentive in a Mixed-Mode Longitudinal Study of Young Adults

This paper describes the impact of a "time-limited" "push-to-web" incentive on response rate and sample composition in a mixed-mode (web, telephone, face-to-face) longitudinal study of young adults in the UK. An "early bird" push-to-web incentive experiment was conducted in the eighth follow-up of the Next Steps cohort study, which follows the lives of a nationally representative sample of around 16,000 people in England born in 1989-90. During the study "soft launch" which tested procedures for the main stage of fieldwork, a randomly allocated group of study members was offered a time-limited & POUND;20 incentive to complete the survey online within three weeks of receiving the study invite (treatment group; n = 1107); the incentive dropped to & POUND;10 after the three-week period and was no longer conditional on mode of completion. The control group was offered a standard & POUND;10 incentive conditional on completing the survey irrespective of mode and time (control group; n = 1108). Here we investigate the impact of the time-limited push-to-web incentive on response rates at three phases of fieldwork: by the end of the time-limited three-week period, by the end of telephone follow-up, and by the end of faceto-face fieldwork (end of fieldwork). We also assess whether the time-limited incentive had a differential impact on subgroups of sample members at any phase of fieldwork, compared to the control group, hence affecting the sample composition. Our analysis shows that the time-limited incentive significantly increased response rates in the treatment group during the first three weeks of fieldwork (27% vs 22%). Response rates in the treatment group remained higher up to the end of telephone follow-up (33% vs 30%), following the withdrawal of the higher value incentive, though this did not reach the conventional level of statistical significance. By the end of fieldwork, however, the time-limited incentive achieved similar response rates to the group offered the standard & POUND;10 incentive (52% vs 53%). The web response rates in the treatment group remained higher throughout fieldwork, but at borderline level of statistical significance. We found no evidence for differential impact of the time-limited incentive on the sample composition in terms of key demographic and survey behaviour characteristics at any fieldwork phase, compared to the control group.