Multisite trials, in which individuals are randomly assigned to alternative treatment arms within sites, offer an excellent opportunity to estimate the cross-site average effect of treatment assignment (intent to treat or ITT) "and" the amount by which this impact varies across sites. Although both of these statistics are substantively…

As investments in K-12 math and science professional development programs expanded over the past decade, researchers and policymakers have questioned whether and how such programs work to improve student learning. This article summarizes the current knowledge-base and offers a theoretical framework researchers can use to design studies that…