Why Random Assignment?

Ever since its founding 40 years ago, MDRC has been known for its use of randomized controlled trials to measure the effects of social and educational policy initiatives. Widely accepted as the “gold standard” of evaluation designs, a randomized controlled trial yields the most robust and credible estimates of a program’s effects because it makes it possible to determine  counterfactual outcomes — that is, what would have happened in the absence of the program?

How does it work? A random assignment design can only be used when there are more potential program participants than there are slots to serve them. In a process akin to a lottery or coin flip, individuals who are eligible for a program are randomly assigned one by one (or in groups) to a treatment group whose members receive the specified intervention or to a control group whose members are embargoed from the program but free to receive other available services. If random assignment is done correctly, the members of both groups share the same readily measureable traits (like gender, education, and economic status) — as well as important characteristics (like motivation) that are harder to measure or even impossible to measure. When the two groups are followed up over time, the differences in their outcomes therefore provide a reliable measure of the program’s effects — or impacts.

Why is this important? By measuring the impact of a program or policy, one is getting a true understanding of its added value over the status quo. A random assignment design also accounts for many kinds of potential bias — including the influence of contextual factors (like economic growth or unemployment rates for a jobs program) or individual motivation (which is particularly important when evaluating voluntary programs).