BAH is based on rental data. Homeowners' monthly mortgage payments are not used in the computation because the monthly cash outlays of homeowners are not a good indicator of the economic costs of home ownership. The variables needed to compute this include such difficult to measure factors as the expected appreciation in the value of the residence, the amount of down payment, the opportunity costs of interest from down payments, settlement costs, and the tax savings due to the interest payments deduction. Therefore, BAH reflects the current rental market conditions not the historical circumstances surrounding various mortgage loans.