A researcher measures serum cholesterol in 25 patients before and after 8 weeks of atorvastatin therapy. The mean difference is 30 mg/dL with SD of differences being 15 mg/dL. The appropriate test to determine if the reduction is statistically significant is:

• A Independent samples t-test with 48 degrees of freedom
• B Chi-square test with 1 degree of freedom
• C Paired t-test with 24 degrees of freedom ✓
• D One-way ANOVA with 1 and 24 degrees of freedom

Explanation

The same subjects are measured twice, making the observations paired or dependent. The paired t-test uses the mean and SD of the differences with df = n - 1 = 24. An independent t-test would be wrong because it ignores the paired nature. Chi-square is for categorical data. ANOVA is for comparing more than two groups.

Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.

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