Differentiate nondifferential from differential misclassification and its impact on bias in cohort studies.

• A. Nondifferential misclassification biases away from the null.
• B. Nondifferential misclassification generally biases toward the null; differential misclassification can bias away from the null in unpredictable directions.
• C. Misclassification never affects bias.
• D. Both types always bias toward the null.

Answer: (B)

In cohort studies, misclassification of exposure or outcome can distort the observed association. Non-differential misclassification means the error in classification is the same across comparison groups (for example, misclassification rates are similar among those who develop the disease and those who do not, or across exposure groups). This pattern tends to pull the measured association toward the null value, diluting any real relationship. In other words, true differences between exposed and unexposed groups look smaller than they actually are.

Differential misclassification happens when the error rate differs by exposure status or by outcome status. Because the misclassification is not uniform, it can distort the association in either direction—and sometimes even reverse it—depending on how those misclassification rates align with the true groups.

So, nondifferential misclassification generally biases toward the null, while differential misclassification can bias away from the null (or toward it) in unpredictable directions.