Recruiting and retaining mobile young injection drug users in a longitudinal study.

Longitudinal studies that research homeless persons or transient drug users face particular challenges in retaining subjects. Between 2005 and 2006, 101 mobile young injection drug users were recruited in Los Angeles into a 2-year longitudinal study. Several features of ethnographic methodology, including fieldwork and qualitative interviews, and modifications to the original design, such as toll-free calls routed directly to ethnographer cell phones and wiring incentive payments, resulted in retention of 78% of subjects for the first follow-up interview. Longitudinal studies that are flexible and based upon qualitative methodologies are more likely to retain mobile subjects while also uncovering emergent research findings.