A case-control study reports an odds ratio (OR) of 3.5 with 95% CI of 1.1–11.2 for the association between pesticide exposure and Parkinson's disease. The wide confidence interval most directly reflects:

• A Systematic bias in exposure measurement
• B Confounding by agricultural occupation
• C Effect modification by age
• D Insufficient statistical power due to small sample size ✓

Explanation

The width of a confidence interval is primarily determined by sample size and the variability of the point estimate. A very wide 95% CI (1.1–11.2 spans tenfold) indicates imprecision arising from inadequate sample size (low statistical power), not systematic bias or confounding — those affect the point estimate's validity, not necessarily its precision. Effect modification would necessitate stratified analysis but would not inherently widen the overall CI.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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