Nine experts rated two brands of the Colombian coffee in the taste-testing experiment. A rating on a seven point scale (1 = very unpleasing, 7 = very pleasing) is given for each and every of four characteristics: taste, richness, aroma, and acidity. The following data (stored in Coffee) display the ratings accrued over all four characteristics.
Brand
Expert
A B
C.C. 24 26
S.E. 27 27
E.G. 19 22
B.L. 24 27
C.M. 22 25
C.N. 26 27
G.N. 27 26
R.M. 25 27
P.V. 22 23
a. At the 0.05 level of significance, is there evidence of the difference in the mean ratings between the two brands?
b. What supposition is necessary regarding the population distribution in order to carry out this test?