External Validity in Policy Evaluations That Choose Sites Purposively

Autor/inn/en	Olsen, Robert B.; Orr, Larry L.; Bell, Stephen H.; Stuart, Elizabeth A.
Titel	External Validity in Policy Evaluations That Choose Sites Purposively
Quelle	In: Journal of Policy Analysis and Management, 32 (2013) 1, S.107-121 (15 Seiten) PDF als Volltext Verfügbarkeit
Sprache	englisch
Dokumenttyp	gedruckt; online; Zeitschriftenaufsatz
ISSN	0276-8739
DOI	10.1002/pam.21660
Schlagwörter	Site Selection; Models; Statistical Bias; Mathematical Formulas; Validity; Policy + Suchen Sie Ihr Suchwort? Analogiemodell; Mathematische Formel; Gültigkeit; Politik
Abstract	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. (As Provided).
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Erfasst von	ERIC (Education Resources Information Center), Washington, DC
Update	2017/4/10