6.9 Refer again to the study described in Problem 6.8. Suppose that the model containing only the “demographic” variables, x3

= annual income and x4

= years of schooling, results in −2ln(likelihood) = 574.

a. Is the difference between the model containing all four predictors and the model containing only the two “demographic” variables significant?

b. Based on these results, which of these models is preferred? What conclusions can be drawn (as related to these variables)? Explain.