Principal Findings
Understanding how to optimize follow-up rates can help researchers design more robust studies and improve data quality. In this case study of adults using e-cigarettes in the fourth wave of an ongoing longitudinal survey, we assessed differences in survey initiation and completion and participant demographics for 2 incentive structures; the conditional structure (US $30 gift code upon survey completion) yielded more survey starts and completions than the hybrid structure (US $15 gift code both before and after survey completion). These findings add to the literature by providing evidence in favor of the conditional structure, consistent with some previous work [Additionally, we find that the conditional and hybrid approaches yielded participants with similar demographics, suggesting that for this case study, one approach was not superior to the other in maintaining the representativeness of the sample. A 2014 study examining participation rates in a web-based questionnaire similarly found no differences by age or gender between conditional and unconditional incentives [