Interpret a 95% confidence interval for a relative risk of 1.5 (95% CI 1.1–2.0): what does it imply?

• A. The true RR equals 1.5 exactly.
• B. The association is not statistically significant because CI excludes 1.
• C. The association is statistically significant; the CI 1.1–2.0 does not include 1.
• D. The result is inconclusive because the CI is wide.

Answer: (C)

The main idea is that a confidence interval for relative risk shows a range of plausible values for the true ratio. Here, the point estimate is 1.5, suggesting a 50% higher risk with exposure, but the 95% confidence interval spans from 1.1 to 2.0. Because this entire interval lies above 1, the association is statistically significant at the 5% level. In other words, the data are consistent with a true relative risk somewhere between 1.1 and 2.0, but not equal to 1. The exact value isn’t known, and the interval could be narrower or wider in future studies, but the fact that it does not include 1 means the result is considered significant. Choosing a value like 1.5 exactly isn’t correct because a confidence interval represents a range of plausible true values, not a single fixed number. If the interval had included 1, the result would not be statistically significant.