Relationship between follow-up rates and treatment outcomes in substance abuse research: more is better but when is "enough" enough?

Aims. To examine the effects of different follow-up rates on estimates of t reatment outcome and predictive models thereof and to specify participant c haracteristics associated with tracking difficulty. Design. An observationa l study using data collected for a randomized, experimental design. Setting . The King County Assessment Center in Seattle, Washington, an organization responsible for referral to publicly funded substance abuse treatment. Par ticipants. Substance-addicted individuals referred to publicly funded inpat ient or outpatient treatment. Measurements. Standardized self-report instru ments measuring substance use, substance use consequences and general funct ioning. Charr review was used to measure treatment entry and completion. Fi ndings. There was a significant association between follow-up difficulty an d outcomes related to addiction treatment and later substance rise. However , outcome estimates based on 60% of the sample who were easiest to locale w ere only minimally different from those based on the 90-100% ultimately cap tured, and predictive models of outcome based on the 60% group were reasona bly similar to those bused on the final sample. Of baseline characteristics examined, only age was associated with later tracking difficulty. Conclusi ons. Studies reporting follow-up rates below 70% may produce valid findings and study attrition may be largely unpredictable from participant characte ristics at baseline. However, a number of factors such as type of populatio n studied geographical location of the sample, reasons for loss to follow-u p and sample size must be considered when attempting to generalize the find ings of this study.