Which statement about nondifferential misclassification is true?

• A. Nondifferential misclassification generally biases away from the null.
• B. Nondifferential misclassification often increases precision but not bias.
• C. Nondifferential misclassification generally biases toward the null.
• D. Nondifferential misclassification always inflates effect size.

Answer: (C)

Nondifferential misclassification of exposure (or outcome) means the misclassification error is independent of the other variable, so the error rates are the same across outcome groups. When errors occur with the same probability in both groups, the true difference between groups gets diluted. In a 2x2 setup, some truly exposed individuals are labeled unexposed and some truly unexposed are labeled exposed, but this misclassification happens equally regardless of whether the disease occurred. The net effect is to pull the observed association toward 1.0, a null effect, because the distinct differences between exposed and unexposed groups are blunted.

For example, if the true risk is higher in the exposed group than in the unexposed group, nondifferential misclassification will move some people across exposure categories in both groups, making the observed risks more similar. The result is a smaller observed risk ratio or odds ratio than the true one.

This is why the statement that nondifferential misclassification generally biases toward the null is the best description. It does not reliably increase precision; random errors usually reduce precision. It also does not universally inflate the effect size; with nondifferential misclassification, the typical pattern is attenuation toward the null, not amplification.