11.71 Refer to Exercise 11.70. The same data are now used to fit the model Thus, we now add the interaction between IQ and SES to the model. The SAS printout for this model is shown in Figure 11.64.

a Identify the least-squares prediction equation.

b Interpret the value of and test to determine whether the data provide sufficient evidence to indicate that this model is useful for predicting achievement test scores.

c Sketch the relationship between predicted achievement test score and IQ for the three levels of SES.

d Test to determine whether these is evidence that the mean increase in achievement test score per unit increase in IQ differs for the three levels of SES.