26. To calculate a confidence interval for an independent-samples t test, we use the: O difference between means. O mean of the second group only. O difference between medians. O mean of the first group only. 27. Which formula for calculating the lower bound of a confidence interval is correct? O t(Smean) + (Mx - My) population O -t(Sdifference) + (Mx - My) sample O -t(Smean) + (Mx - My) population O t(Sdifference) + (Mx - My) sample