A manufacturer of concrete bridge supports is interested in determining the effect of varying the sand content of concrete on the strength of the supports. Five supports are made for each of five...

A manufacturer of concrete bridge supports is interested in determining the effect of varying the sand content of concrete on the strength of the supports. Five supports are made for each of five different amounts of sand in the concrete mix and each support tested for compression resistance. The results are as shown in Table 6.27.

(a) Perform the analysis to determine whether there is an effect due to changing the sand content.

(b) Prior to the experiment, the researchers planned to compare the average of the means in the “medium” groups (20, 25, 30) with the mean in the “low sand” group (15), and with the mean in the “high sand” group (35). Test these two contrasts. Assume that these are not necessarily the only contrasts, but that the researchers will freely explore other ideas based on the data.