A 95% confidence interval for a mean difference is 2.1 to 5.7. At the corresponding two-sided 5% significance level, which conclusion follows?

Correct answer: The null hypothesis of zero difference is rejected. The interval 2.1 to 5.7 does not include zero, so the null hypothesis of zero difference is rejected at the 5% level.

• The population difference is guaranteed to lie inside the interval. A confidence interval does not guarantee that the population value lies inside it.
• The two sample means must have been equal before analysis. The sample means differ, as the interval shows a positive difference.
• The result proves practical importance in every educational setting. Statistical significance does not prove practical importance.