Some Lessons From 50 Years of Multiarm Public Policy Experiments.

In this article, we explore the reasons why multiarm trials have been conducted and the design and analysis issues they involve. We point to three fundamental reasons for such designs: (1) Multiarm designs allow the estimation of "response surfaces"-that is, the variation in response to an intervention across a range of one or more continuous policy parameters. (2) Multiarm designs are an efficient way to test multiple policy approaches to the same social problem simultaneously, either to compare the effects of the different approaches or to estimate the effect of each separately. (3) Multiarm designs may allow for the estimation of the separate and combined effects of discrete program components. We illustrate each of these objectives with examples from the history of public policy experimentation over the past 50 years and discuss some design and analysis issues raised by each, including sample allocation, statistical power, multiple comparisons, and alignment of analysis with goals of the evaluation.