Assessing Evaluation Studies The Case of Bilingual Education Strategies (1992) / Chapter Skim
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2 Research Methodology and Bilingual Education
2 Research Methodology and Bilingual Education
Pages 12-23
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From page 12... ...
one is claiming, at least in a loose sense, that X "causes" Y The notion of which designs allow conclusions about the causal effects of treatments is critical to an appreciation of He evaluation of 12
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As part of the development of the evidential base leading to a claim that X causes Y it is useful to distinguish between confirmation studies, whose purpose is to confirm a prior hypothesis, and discovery studies, whose purpose is to discover candidate mechanisms.
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This probability can be determined in advance. In this sense, sample surveys allow for external validity, that is, for the generalization from sample to population.
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The two are essentially equivalent concepts, and we use them interchangeably, depending on the context.) Observational Studies Sample surveys are a form of observational study involving a collection of cases.
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Alternatively, one could determine the intervention effect through random assignment to Me intervention: see Me role of random assignment in experiments below. Analyses that takes explicit account of Me process of self-selection into public versus private schools, or more general selection processes, are an essential ingredient of assessments (in observational studies)
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Quas~-exp~enments allow for stronger causal inferences than do uncontrolled observational studies. They fall short of randomized controlled experiments as the study of choice for causal purposes.
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That is, in this context, randomization or random selection from a well-defined population provides external validity. Experiments use random assignment to treatments in drawing inferences about
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Thus, they would accept external validity in a population similar in important respects to the one studied, even if the sample units were not selected at random from that population. They might also accept internal validity for a study that could be argued to resemble a randomized experunent or for which they could build a defensible statistical model of the differences from a randomized experiment and use it to adjust for those differences.
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If one cames out the assignment of treatments at the level of schools, then that is the level that can be justified for causal analysis. To analyze the results at the student level is to introduce a new, nonrandomized level into the study, and it raises the same issues as does the nonrandomized observational study.
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Research studies attempt to determine explicitly the relative effectiveness of specifically defined programs. Investigators often conduct studies in restricted populations and attempt to use experimental design features, such as randomization and matching.
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From page 23... ...
(1965) The plug of observational studies of human populations (with discussions.
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