If a 95% confidence interval for a risk ratio includes 1.0, what does this imply?

• A. The association is statistically significant at the 0.05 level.
• B. There is a statistically significant association but with uncertainty.
• C. The data show a definite effect.
• D. The association is not statistically significant at the 0.05 level; we cannot rule out no effect.

Answer: (D)

A 95% confidence interval for a risk ratio that includes 1.0 means the null value of no effect is still plausible given the data. Because 1 lies within the interval, we cannot claim a statistically significant association at the 0.05 level, and the p-value would be greater than 0.05. Confidence intervals reflect sampling variability, so if the study were repeated many times, about 95% of the intervals would contain the true effect. If the entire interval were above 1 or entirely below 1, that would indicate a statistically significant increased or decreased risk, respectively. Here, including 1 indicates we cannot rule out no effect.