15.71 Heavy Metal An experiment was performed to determine whether there is an accumulation of heavy metals in plants that were grown in soils amended with sludge and whether there is an accumulation of heavy metals in insects feeding on those plants. The data in the table are cadmium concentrations (in g/kg) in plants grown under six different rates of application of sludge for three different harvests. The rates of application are the treatments. The three harvests represent time blocks in the two-way design. 

a. Based on the MINITAB normal probability plot and the plot of residuals versus rates, are you willing to assume that the normality and constant variance assumptions are satisfied?

b. Using an appropriate method of analysis, analyze the data to determine whether there are significant differences among the responses due to rates of application.

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Harvest Rate 1 2 3 Control 1 162.1 153.7 200.4 199.8 199.6 278.2 2 220.0 210.7 294.8 3 194.4 179.0 341.1 4 204.3 203.7 330.2 5 218.9 236.1 344.2