If the 95% CI for a SIR is 2.1-4, what does this imply about the null value of 1?

• A. The CI includes 1 (not significant)
• B. The CI excludes 1 (statistically significant)
• C. The CI cannot be determined
• D. The CI is irrelevant

Answer: (B)

A confidence interval for a SIR shows the range in which the true ratio of observed to expected cases is likely to lie. The null value is 1, meaning no difference between what was observed and what was expected. If the entire interval sits above 1, you can reject the null and conclude a statistically significant excess. Here, the interval ranges from 2.1 to 4, which is entirely above 1, so we are 95% confident that the true SIR is greater than 1. This indicates a significantly higher incidence than expected, not a non-significant result or an indeterminate one.