Abstract
In a comparison of a treatment group and a control group, a difference in results can correspond to the effect of the treatment; however, the difference might also be, at least in part, a reflection of pretreatment differences between the two populations. Covariance adjustment can reduce bias in the estimate of the treatment effect ( Cochran & Rubin, 1973 ); however, baseline group differences with respect to unobserved covariates which cannot be controlled can lead to hidden bias. This article presents a simple method which uses omitted variable bias to assess the uncertainty of the hidden bias by describing what scenarios regarding the unobserved covariate can lead to a given level of hidden bias. A comparison of a culturally sensitive AIDS video education program and a standard AIDS video education program is used as an illustration.
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