In a case-control study of a vaccine and Guillain-Barre syndrome, vaccination status was verified from computerized pharmacy records for both cases and controls, so exposure misclassification occurred equally in both groups. Assuming no other flaw, the resulting odds ratio will most likely:

• A Overestimate the association in either direction unpredictably
• B Be biased away from the null value
• C Remain unbiased if the sample size exceeds 500
• D Be biased toward the null value ✓

Explanation

When misclassification of exposure is non-differential, meaning it occurs independently of disease status in both cases and controls, the effect estimate is diluted toward the null. An apparent association may disappear, but a spurious association will not be created. Large sample size reduces random error only and cannot correct systematic dilution, so option C fails. Differential misclassification is what produces unpredictable swings in either direction.

Reference: Gordis Epidemiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.