15. State the critical values for a two-independent-
sample t test given the following conditions:
(a) Two-tailed test, \( \alpha = .01 \), total \( df = 26 \)
(b) One-tailed test, lower-tail critical, \( \alpha = .01 \),
\( df = 15 \) for each group
(c) Two-tailed test, \( \alpha = .05 \), \( n = 12 \) in each
group
(d) One-tailed test, upper-tail critical, \( \alpha = .05 \),
\( n \) for both groups combined is 30