Interpreting treatment effects when cases are institutionalized after treatment.

Drug treatment clients are at high risk for institutionalization, i.e., spending a day or more in a controlled environment where their freedom to use drugs, commit crimes, or engage in risky behavior may be circumscribed. For example, in recent large studies of drug treatment outcomes, more than 40% of participants were institutionalized for a portion of the follow-up period. When longitudinal studies ignore institutionalization at follow-up, outcome measures and treatment effect estimates conflate treatment effects on institutionalization with effects on many of the outcomes of interest. In this paper, we develop a causal modeling framework for evaluating the four standard approaches for addressing this institutionalization confound, and illustrate the effects of each approach using a case study comparing drug use outcomes of youths who enter either residential or outpatient treatment modalities. Common methods provide biased estimates of the treatment effect except under improbable assumptions. In the case study, the effect of residential care ranged from beneficial and significant to detrimental and significant depending on the approach used to account for institutionalization. We discuss the implications of our analysis for longitudinal studies of all populations at high risk for institutionalization.