It is commendable that the sample size for the entire comparison group study is large enough (about 3,000 subjects) and therefore may have substituted for the internal validity which is achievable in experimental studies even when small samples are used. In the selection process for the comparison group design, important factors such as the region where students were studying were not a major consideration yet the environment (rural or urban) may have determined the outcome variables. It is also important to mention that the propensity-score method being a non-experimental method; failed to determine the effects of observed and non-observed factors on the outcome. As such, there is no firm basis for comparing the experimental and propensity-score methods outcomes. This study also ignores confounding variables such as an individual’s motivation which may affect the outcomes of the study.

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Effects of observed and non-observed factors on the outcome
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