In the Park Test, one of the major difficulties is choosing a proportionality factor (Z). In each of the following equations, seperate the listed explanatory variables into those that are likely or unlikely to be proportionality factors.
a. The # of econ majors in a cross section of various-sized colleges and universities as a function of the number of undergraduates attending a school, the # of required courses in that school's econ major, the average GPA in the major, and the # of economics professors there.
b. GDP in a cross section of various countries as a function of the aggregate gross investment in a nation, the % growth of its money supply, the maximum marginal tax rate on capital gains there, and its population.
c. The demand for carrots in a time-series model of the US as a function of the real price of carrots, US per capita disposable income, population, the % error in carrot sales measurement, and the real price of celery.