The 95% confidence interval is providing a range that you are 95% confident the true difference in means falls in. Thus, the CI can include negative numbers, because the difference in means may be negative.

What does a negative CI mean?

1 Nov 2010, 15:55. Several questions here : (1) Meaning of a negative CI : A negative confidence lower confidence limit suggests the use of an approximate method for calculating the standard error usually in combination with a small sample size.

Is confidence level always positive?

If a test of the difference is significant, then the direction of the difference is established because the values in the confidence interval are either all positive or all negative.