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Two common methods of measuring personality variables are examined with respect o the ways in which they control respondent biases. A form of the mental-test theory model is developed which makes explicit differences in score components hat result from different methodological constraints im-posed by the two methods. From the model, it is possible to specify the oper-ations necessary to provide equivalent information from the two sets of data. Performance of these operations i an empirical test supports the model by producing generally higher correlations between the variables having score components which are more closely matched. Under one form of the mental-test theory model, an individual's core on a personality test can be considered to be a function of three independent types of components: the true status of the individual on the attribute the test is intended to measure, other systematic sources, and a random error component. In recent years, there has been a growing interest in systematic effects on test scores deriving both from subject characteristics other tha