Random Assignment in Program Evaluation and Intervention Research: Questions and Answers

Why random assignment instead of 
another approach?
Researchers sometimes use approaches other than
random assignment in an impact study to determine the
effect of a new program. One example of an alternative
approach is a simple “pre-post” comparison. Here, using
our new math program again, the study would compare
students’ math test scores before they participated in the
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Is participation in the evaluation voluntary?
In some cases, participation in a study may be
voluntary, and in others, it may be required as a
consequence of a school district or school trying out a 
new educational practice. For example, if researchers are
examining the effectiveness of an afterschool program,
students’ participation in the study may be voluntary—
parents can decide whether their child will attend the
program. But when researchers are examining the
effectiveness of different teaching methods, such as
variations on how technology may be integrated into the
classroom, schools may randomly assign one variation or
another to entire classrooms. In this case, participation is
“required” in that students may not have an option to
change classes. Similarly, school districts that want to test
alternative whole-school reforms may decide that all
schools should participate in the evaluation. In all cases,
however, the evaluator should work closely with school
districts, schools, teachers, students, and parents to ensure
that everyone has a sound understanding of the importance
of the study and the study procedures.
Does the intervention group need to have the 
same number of participants as the control group?
Some variation in the number of participants in the
two groups can be accommodated. For example, a 
study of the impact of offering a new type of professional
development for teachers might have 200 teachers