What is the correct interpretation when a confidence interval includes the null value?

• A. If the confidence interval includes the null value, the result is not statistically significant.
• B. If it includes null, the result is statistically significant.
• C. Including null guarantees a large effect.
• D. It has no bearing on statistical significance.

Answer: (A)

When a confidence interval for the estimated effect includes the null value, it means there isn’t enough evidence to declare a statistically significant effect at the chosen confidence level. The interval represents the range of true effects that are plausible given the data; if the value representing “no effect” lies inside that range, you cannot reject the possibility of no effect based on this study. In practical terms, the p-value is above the alpha threshold for significance at this level. This doesn’t prove there is no effect; it just indicates the data aren’t precise enough to show a statistically significant one. If the interval had excluded the null, that would indicate statistical significance at the same level.