In a study, the 99% confidence interval for the mean is reported as wider than the 95% confidence interval calculated from the same data. This is because:

• A The standard deviation increases with higher confidence level
• B The standard error increases with confidence level
• C The sample size effectively decreases
• D The critical value (z) increases to capture more of the sampling distribution ✓

Explanation

A 99% CI uses a larger critical value (z=2.58) than a 95% CI (z=1.96) to capture more of the normal curve, making the interval wider. The standard deviation and standard error are properties of the data and sample, not of the confidence level chosen. Sample size is fixed. This tests whether students understand what drives CI width.

Reference: Basic and Clinical Biostatistics, 4th ed.

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