Abstract
Masking evaluators is a complex process, calling upon the creativity of scientific investigators and the unrelenting attention of project directors and data collectors. This article reviews research supporting the importance of masking to reduce the effects on outcome of evaluator bias, especially given the evidence of strongly held beliefs about the effects of prenatal cocaine exposure. We discuss the particular difficulty of totally masking examiners who are blind to drug status yet aware of the nature of the study, given an association of drug use and sociodemographic and lifestyle conditions that evaluators may know or observe. Finally we suggest techniques that may be used to facilitate masking. As part of a longitudinal study of the effects of prenatal cocaine exposure, we asked blinded evaluators, after completing the Brazelton Neonatal Behavioral Assessment Scale, to guess whether or not they thought each infant had been cocaine-exposed. The 276 infants were evaluated at birth in a research room without their parents so that the only information available to examiners was what could be observed about the infant. Those infants whom examiners guessed to be cocaine-exposed were significantly smaller by birthweight, were more likely to be black, and had poorer Brazelton performance. The guesses were correct slightly more often than chance (56 percent; p = .050) because of an actual, though weak, relationship between size and performance. In short, the guesses of examiners demonstrated a bias about the relationship of poorer performance and cocaine exposure, supporting the critical importance of our efforts to maintain masking in evaluations.
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