27. Suppose, in Problem 23, that there has been some controversy about the assumption of no interaction between gasoline and additive used. To allow for the possibility of an interaction effect between gasoline and additive, it was decided to run 36 motors — 4 in each grouping. The following data resulted.

a. Do the data indicate an interaction effect?

b. Do the gasolines appear to give equal results?

c. Test whether or not there is an additive effect or whether all additives work equally well.

d. What conclusions can you draw?

Gasoline Additive 1 2 3. 1 126.2 130.4 127 124.8 131.6 126.6 125.3 132.5 129.4 127.0 128.6 130.1 2 127.2 142.1 129.5 126.6 132.6 142.6 125.8 128.5 140.5 128.4 131.2 138.7 3 127.1 132.3 125.2 128.3 134.1 123.3 125.1 130.6 122.6 124.9 133.0 120.9