This paper describes several common types of research studies in special education transition literature and the threats to their validity. It then describes how the evidential base may be broadened, how diverse sources of evidence can be combined to strengthen causal inferences, and the role of judgment within quasi-experimentation. The paper…

The Walsh and Carballo evaluation of the effectiveness of transitional bilingual education programs in five Massachusetts communities, has the following flaws: (1) the sample of school districts studied suffered from "self-selection bias"; (2) the sample does not include a single large, urban school district; (3) the student samples…