Tracking and Locating Itinerant Subjects with a Rechargeable Incentive Card: Results of a Randomized Trial

High attrition among research participants undermines the validity and generalizability of field research. This study contrasted two incentivizing methods-money orders (MOs) or rechargeable incentive cards (RICs)-with regard to rates of participants' study engagement and follow-up contact over a 6-month period. Methods: Substance abusers (N = 303) in Los Angeles, California were recruited and randomized to either an MO (control) or RIC (experimental) condition. All participants were asked to call the researchers at the beginning of each calendar month for the ensuing 5 months to update their locator information, even if nothing had changed. Each call resulted in a $10 payment, issued immediately via the RIC system or by MO by mail. Research staff located and interviewed all participants at Month 6. Contact logs assessed level of effort required to locate participants and conduct follow-up interviews. Results: Relative to controls, RIC participants, especially those with low ability to defer gratification, initiated more monthly calls. Six-month follow-up rates did not differ between RIC (75%) and controls (79%), though the RIC condition was associated with an average staff time savings of 39.8 minutes per study participant. Discussion: For longitudinal public health research involving itinerant study participants, the RIC method produces a modest benefit in study engagement and reduced staff time devoted to participant tracking and payments. However, the overall cost-effectiveness of this approach will depend on the pricing model of the card-issuing vendor (which in turns depends on the scale of the project, with per-unit costs falling for larger projects).