Baseline, Placebo, and Treatment: Efficient Estimation for Three-Group Experiments
Political Analysis, Vol. 18, No. 3, pp. 297-315, 2010
Posted: 5 Jan 2011
Date Written: May 1, 2010
Randomized experiments commonly compare subjects receiving a treatment to subjects receiving a placebo. An alternative design, frequently used in field experimentation, compares subjects assigned to an untreated baseline group to subjects assigned to a treatment group, adjusting statistically for the fact that some members of the treatment group may fail to receive the treatment. This article shows the potential advantages of a three-group design (baseline, placebo, and treatment). We present a maximum likelihood estimator of the treatment effect for this three-group design and illustrate its use with a field experiment that gauges the effect of prerecorded phone calls on voter turnout. The three-group design offers efficiency advantages over two-group designs while at the same time guarding against unanticipated placebo effects (which would undermine the placebo-treatment comparison) and unexpectedly low rates of compliance with the treatment assignment (which would undermine the baseline-treatment comparison).
Keywords: Field Experiment, Three-Group Design, Voter Turnout
JEL Classification: C93, C00
Suggested Citation: Suggested Citation