In the case of the data on scab disease quoted in Section 6.5, find a contrast measuring the effect of the season in which sulphur is applied and give an appropriate HDR for this contrast.

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6.5.5 Example Cochran and Cox (1957, Section 4.13) quote the following data from an experiment on the effect of sulphur in reducing scab disease in potatoes. In addition to untreated plots which serve as a control, three amounts of dressing were compared: 300, 600 and 1200 pounds per acre. Both an autumn and a spring application of each treatment were tried, so that in all there were seven distinct treatments. The effectiveness of the treatments were measured by the 'scab index', which is (roughly speaking) the