Suppose a 95% confidence interval for the mean difference in blood pressure between a treatment group and a control group is $(-2,5)$. What does this confidence interval suggest about the effectiveness of the treatment?

• A. The interval is entirely below $0$, so the treatment is proven to be ineffective.
• B. The confidence interval shows that the treatment always increases blood pressure.
• C. The interval is entirely above $0$, so the treatment is proven to be effective.
• D. The interval includes $0$, so there is no statistically significant evidence that the treatment is effective.

Answer: (D)

Verified step by step guidance

1. Understand that a 95% confidence interval for the mean difference in blood pressure represents a range of values within which we are 95% confident the true mean difference lies.
2. Note that the given confidence interval is (-2, 5), which means the interval includes zero.
3. Recall that if zero is included in the confidence interval for a mean difference, it suggests that there is no statistically significant difference between the treatment and control groups at the 95% confidence level.
4. Interpret this to mean that the treatment effect could be negative, positive, or zero, so we cannot conclude that the treatment is definitively effective or ineffective based on this interval.
5. Therefore, the confidence interval suggests that there is no statistically significant evidence that the treatment changes blood pressure compared to the control.