If a confidence interval includes zero, what can be concluded about statistical significance?

• A. It is statistically significant
• B. It is not statistically significant
• C. P-value is below 0.05
• D. It indicates a large effect size

Answer: (B)

Interpreting confidence intervals and statistical significance: when a confidence interval for an effect estimate includes zero, zero is a plausible value for the true effect at the chosen level of confidence. That means we cannot rule out no effect based on the data, so the result is not statistically significant at that level. The same idea is reflected in the p-value: it would be greater than the alpha threshold (for example, above 0.05 for a 95% CI). This does not prove there is no effect—it simply indicates that, with this sample and variability, the data do not provide strong enough evidence to conclude a nonzero effect. A wide interval often signals imprecision due to small sample size or high variability, while a narrow interval that excludes zero would indicate statistical significance.