Bias despite masked assessment of clinical outcomes when an outcome is defined as one of several component events.

Hypothetical examples are presented that show that bias can result from misclassification of clinical outcomes defined as one of several events, even though the assessment of each component event was masked. In one example, the effect of the overall misclassification is to make a treatment that reduces the risk appear to increase it; in another example, misclassification causes the overall treatment effect to appear stronger than it actually is. These anomalies are due to the fact that the misclassification of the overall outcome can be differential, even though the misclassification of the individual components is nondifferential.