In a survey of 9,966 economists in 1964 the following data were obtained:
a. Develop a suitable regression model to explain median salary in relation to age. For the purpose of regression, assume that median salaries refer to the midpoint of the age interval.
b. Assuming error variance proportional to age, transform the data and obtain the WLS regression.
c. Now assume that it is proportional to the square of age. Obtain the WLS regression on this assumption.
d. Which assumption seems more plausible?