What is Random Assignment?

Researchers use random assignment in impact studies to form two statistically equivalent groups of participants in the most objective way possible. The term "participants" may refer to students, teachers, classrooms, or schools.

Random assignment procedures vary according to the program being tested. In most situations, the basic process of randomly assigning participants to an education intervention and a control condition are similar. First, a list of participants is created. The list of participants is then randomly assigned to conditions. Some of the participants are assigned to the program condition, and the other participants are assigned to the control condition. The actual number of participants assigned to the program and control conditions, carefully thought out by the evaluator, is based on statistical considerations such as the "size" of the impact one wants to detect and the chances of observing large impacts by chance.

For instance, in a random assignment impact study of our hypothetical math program, classrooms would be the participants, and they would be selected in a purely random way to be part of one of two conditions. One group of classrooms would have the new math program in its curriculum — this is known as the "intervention" or "program" condition. The other group of classrooms would continue in the school’s regular math program — this is known as the "control" condition.

On the other hand, if a school system wants to try out a new method of professional development for teachers to see whether it changes teaching practices in a way that helps students score higher on achievement tests, teachers would be the participants who are randomly assigned to the intervention or the control condition. The former would be exposed to the new professional development program, and the latter would be offered the regular program.

In yet another example of random assignment, suppose a state wants to assess different comprehensive school reform strategies as the avenue to better test scores. In this case, schools would be the participants randomly assigned to the various reform conditions (i.e., interventions) or to the control condition. In other words, the reforms would be put into place in some schools, but not in others, and random assignment would be used to select the schools to implement the reforms.

The assignment process works much the same as a lottery, so each participant — whether a student, a classroom, a teacher, or a school — has the same chance of ending up in the intervention or the control group. For instance, in our hypothetical math program, imagine that each classroom is given a number on a slip of paper. The slips of paper are then placed in a bin and shuffled, and half the numbers are pulled from the bin. Classrooms with these numbers are assigned to the new math program (intervention condition), and classrooms with numbers remaining in the bin continue in the regular program (control condition). In practice, the assignment process is performed by a computer.

Because classrooms are assigned to one condition or the other wholly at random, the chances of a more or less capable, or a more or less motivated classroom of students ending up in one condition or the other are the same. So the only difference between the two conditions is the type of math instruction they are getting. This "one-difference-only" feature is the main advantage of using random assignment in an impact study, because it means that if the math skills of the students in the new program improve more than the math skills of the students in the regular program, we can be almost completely certain that the improvement occurs because of the program, not because of student ability or motivation or other factors that might influence their achievement.