By creating and analyzing matched samples, researchers can simplify their analyses to include fewer covariate variables, relying less on model assumptions, and thus generating results that may be easier to report and interpret. When two groups essentially "look" the same, it is easier to explore their differences and make comparisons…

The present paper describes the recruitment and site selection process that has been underway since January 2011, with particular emphasis on the use of Mahalanobis distance score to determine matched pairs of sites prior to randomization to treatment and control groups. Through a systematic winnowing process, the authors found that they could…

This report summarizes the complex research literature on quantitative methods for assessing how impacts of educational interventions on instructional practices and student learning differ across students, educators, and schools. It also provides technical guidance about the use and interpretation of these methods. The research topics addressed…

Randomized controlled trials in educational research tend to be small. Small trials can have large, chance, imbalances in important covariates. For studies with sample sizes greater than 50, chance imbalances can be corrected using analysis of covariance; for small trials, however, statistical power is maximized if the trial is balanced and…

This article considers various kinds of probability and non-probability samples in both experimental and survey studies. Throughout, how a sample is chosen is stressed. Size alone is not the determining consideration in sample selection. Good samples do not occur by accident; they are the result of a careful design. (Author/JAZ)

It is difficult to conduct randomized field experiments. In the past decade, the use of alternative randomization plans and incentives has contributed to their operational feasibility; legal, ethical, and professional arguments for experimentation have matured; and expectations have become better aligned with practical constraints that are likely…

Recruiting schools and kids to participate in drug prevention evaluations is no longer, if it ever was, a "done deal." Since the mid-1980s, schools have been inundated with requirements for drug programs and requests to cooperate with program evaluations. As a consequence, both treatment and control schools are harder to find. In addition, many…

Research efforts have established that the Mantel-Haenszel procedure (MHP) is an effective method for detecting the presence of test items exhibiting differential item functioning (DIF). While the MHP has been advocated for situations where item response theory based methods may not be usable, recent findings have suggested that the performance of…