Let C be total consumption expenditures of a family, and S the size of the family. Formulate an econometric model that the marginal effect of family size on consumption is a constant. What is the expected sign of the relevant coefficient? Formulate an econometric model that the marginal effect of family size on consumption decreases as S increases (because of economies of scale in cooking, rents and so on). What are the expected sign(s) of the relevant coefficient(s)?