I recently computed an independent t test (2-tailed test) question in Spss using the parameters below:
H0: There's no difference in the mean scores of a maths quiz taken by students in class A and B.
H1: There's a difference in the mean scores of a maths quiz taken by students in class A and B.
Class A: n = 70, mean = 4.3226, SD = 1.11731, SEM = 0.13354
Class B: n = 269, mean = 4.4117, SD = 1.14620, SEM = 0.06988
The 95% CI calculated was (-0.39006,0.21188)
However,when I swap the 2 groups around, the 95% CI becomes (-0.21188,0.39006).
While I understand that the range is the same, the values aren't, and there's an overlap between the 2 CIs. Which CI do I use?
The difference is one minus the other. Which term comes first in a subtraction affects the sign of the difference though not the magnitude
Your first confidence interval of $(-0.39006,0.21188)$ is for the difference in the population means $\mu_A- \mu_B$
Your second confidence interval of $(-0.21188,0.39006)$ is for $\mu_B- \mu_A$ so is $-1$ times the first
Since $0$ is in the confidence interval (either), you presumably will not be rejecting the null hypothesis