In a case-control study of oral cancer, investigators individually match each case with a control of identical age, sex and tobacco-chewing status. A consequence of matching on tobacco chewing is:

• A Tobacco chewing can still be evaluated as an independent risk factor
• B Matching eliminates the need for multivariate adjustment of other confounders
• C Matching always strengthens the precision of every exposure estimate
• D The matched variable can no longer be studied as a risk factor in its own right ✓

Explanation

Once cases and controls are forced to be identical on a characteristic, that variable has no contrast left between groups, so its independent effect on disease cannot be estimated; this is overmatching. Matching also consumes statistical efficiency if the matched factor is not truly a confounder, so it does not automatically sharpen every estimate, and residual confounding by unmatched variables still requires adjustment. Tobacco chewing's effect becomes unmeasurable here.

Reference: Modern Epidemiology (Rothman), 4th ed.

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