The Taylor Series Aproximationfor FAS 91 Adjustments

Financial Accounting Standard Number 91 deals with the calculation of investment income and amortized cost for mortgage backed securities, as well as whole mortgages and callable bonds. Complying with the standard's treatment of mortgage backed securities has proven to be difficult, partly because it requires the company to keep a record of all past cash flows in order to calculate the accrual of discount or amortization of premium. The method described here uses a simple Taylor series to eliminate the storage of all past cashflows, while still providing a very accurate FAS 91 adjustment.