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Evaluations of the impact of social programs are often carried out in multiple “sites,” such as school districts, housing authorities, local TANF offices, or One-Stop Career Centers. Most evaluations select sites purposively following a process that is nonrandom. Unfortunately, purposive site selection can produce a sample of sites that is not representative of the population of interest for the program. In this paper, we propose a conceptual model of purposive site selection. We begin with the proposition that a purposive sample of sites can usefully be conceptualized as a random sample of sites from some well-defined population, for which the sampling probabilities are unknown and vary across sites. This proposition allows us to derive a formal, yet intuitive, mathematical expression for the bias in the pooled impact estimate when sites are selected purposively. This formula helps us to better understand the consequences of selecting sites purposively, and the factors that contribute to the bias. Additional research is needed to obtain evidence on how large the bias tends to be in actual studies that select sites purposively, and to develop methods to increase the external validity of these studies.

What housing and service interventions work best to reduce homelessness for families in the United States? The FamilyOptions Study randomly assigned 2,282 families recruited in homeless shelters across 12 sites to priority access to one of three active interventions or to usual care in their communities. The interventions were long-term rent subsidies, short-term rent subsidies, and transitional housing in supervised programs with intensive psychosocial services. In two waves of follow-up data collected 20 and 37 months later, priority access to long-term rent subsidies reduced homelessness and food insecurity and improved other aspects of adult and child well-being relative to usual care, at a cost 9 percent higher. The other interventions had little effect. The study provides support for the view that homelessness for most families is an economic problem that long-term rent subsidies resolve and does not support the view that families must address psychosocial problems to succeed in housing. It has implications for focusing government resources on this important social problem. C 2018 by the Association for Public Policy Analysis and Management.

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