Thirty-three men with high serum cholesterol, all regular coffee drinkers, participated in a study to see whether abstaining from coffee would affect their cholesterol level. Twenty-five of the men (chosen at random) drank no coffee for 5 weeks, while the remaining 8 men drank coffee as usual. The accompanying table shows the serum cholesterol levels (in mg/dl) at baseline (at the beginning of the study) and the change from baseline after 5 weeks.24

For the following t tests use non directional alternatives and let α = 0.05.
(a) The no-coffee group experienced a 35 mg/dl drop in mean cholesterol level. Use a t test to assess the statistical significance of this drop.
(b) The usual-coffee group experienced a 26 mg/dl rise in mean cholesterol level. Use a t test to assess the statistical significance of this rise.
(c) Use a t test to compare the no-coffee mean change (-35) to the usual-coffee mean change (+26).

No coffee (n 25) Usual coffee (n 8) Mean SD 37 Mean 331 SD 30 Baseline 341 Change 35 from baseline -3527 +26 56