Tracking substance abusers in longitudinal research: understanding follow-up contact difficulty.

Studies examining follow-up contact difficulty provide useful information for planning longitudinal studies and for assessing the validity of follow-up data. Contact difficulty was examined among 96 substance abusers following substance abuse treatment. Interview completion rates at the 3-month and 6-month follow-ups were 93 and 97%, respectively. The extent of contact efforts required to complete follow-up interviews varied substantially but tended to be greater at the 3-month follow-up than at the 6-month follow-up. Contact difficulty was related to reuse of substances at the 3-month and at the 6-month follow-ups with reusers requiring greater contact efforts than abstainers. None of the baseline individual and contextual variables examined significantly predicted level of contact effort at follow-ups. Attrition-related validity implications are discussed along with practical suggestions for planning tracking efforts.