relevant conditions or attributes. As described in the previous chapter, randomization greatly increases the likelihood of being able to infer that an observed difference between the treatment and control groups is causal. Observing a difference in outcome between the groups of participants can be the basis for a causal inference. In controlled field experiments, researchers analyze the results of a deliberately manipulated factor of interest, such as the race of an interviewer. They attempt to control carefully for any intervening or confounding variables. Random assignment of treatments to participants is frequently used to reduce any doubts about lingering effects of unobserved variables, provided, of course, that one can actually apply the randomization to the variable of interest.
In addition to the problem of credibly designing an experiment that supports a causal inference, a common weakness of experiments is a lack of external validity. That is, the results of the experiment may not generalize to individuals other than those enrolled in the experiment, or to different areas or populations with different economic or sociological environments, or to attributes that differ from those tested in the experiment.
Despite these problems, the strengths of experiments for answering some types of questions are undeniable. Even if their results may not be completely generalizable and even if they do not always capture all the relevant aspects of the issue of interest, experiments provide more credible evidence than other methods for measuring the effects of an attribute (e.g., race) in one location and on one population.
Use of an experimental design to measure racial discrimination raises important questions because race cannot be directly manipulated or assigned randomly to participants. Researchers who use randomized controlled experiments to measure discrimination, therefore, can manipulate race by either varying the “apparent” race of a target person as the experimental treatment or can manipulate “apparent” discrimination by randomly assigning study participants to being treated with different degrees of discrimination.
In the first case, the experimenter varies the treatment, namely, the apparent race, by such means as by providing race-related cues on job applications (e.g., name or school attended) or by showing photographs to participants in which the only differences are skin color and facial features. The experimenter then measures whether participants respond differently under one race treatment compared with another (e.g., evaluating black versus white job applicants or associating positive or negative attributes with photographs of blacks versus whites). In such a study, the experimenter elicits responses from the participants to determine the effect of