The following model is a simplified version of the multiple regression model used by Biddle and Hamermesh (1990) to study the tradeoff between time spent sleeping and working and to look at other factors affecting sleep: sleep = β0 + β1 totwrk + β2 educ + β3 age + β4 age2 + β5 male + u where sleep and totwrk (total work) are measured in minutes per week and educ and age are measured in years. Male is a dummy variable for gender (male=1). The regression results are as follows (with standard errors in parentheses).
(d) Discuss the sign and magnitude of the estimated coefficient on educ.