4. An economist is analyzing the relationship between disposable income and food expenditure for the citizens of a developing country. A survey of the citizens has been taken, and data have been collected on income and food expenditures. The following statistical model is postulated for the data:

Yi = fllz,?e€i, where Yi = expenditure on food for the ith household, 2i = disposable income for the ith household, e;s ~ iid N(O, a2 ).