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

• A. The interval is entirely below $0$, so the treatment is definitely not effective.
• B. The interval is entirely above $0$, so the treatment is definitely effective.
• C. The interval includes $0$, so there is not enough evidence to conclude the treatment is effective.
• D. The confidence interval proves the treatment has no effect.

Answer: (C)

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 the given confidence interval is (-2, 5), which means the interval includes zero.
3. Recall that if the confidence interval includes zero, it suggests that the true mean difference could be zero, indicating no effect of the treatment.
4. Interpret that because zero is within the interval, we do not have sufficient evidence to say the treatment has a statistically significant effect on blood pressure.
5. Conclude that the correct interpretation is: the interval includes zero, so there is not enough evidence to conclude the treatment is effective.