Randomized experiments are often seen as the "gold standard" for causal research. Despite the fact that experiments use random assignment to treatment conditions, units are seldom selected into the experiment using probability sampling. Very little research on experimental design has focused on how to make generalizations to well-defined…

The Primary Reading Program was initiated in 117 classrooms in 40 North Carolina schools, to improve the reading achievement of the primary school students. The same number of control classrooms were also selected for comparison. Features of the program included increased funding for instructional materials; use of volunteers and teacher aides for…