approximately w% of the remaining mortgage balance at the beginning of the month, less the scheduled prin- cipal payment, will prepay that month. That is,   Prepayment for month t = SMM ´ (Beginning mortgage balance for month t - Scheduled principal payment for month t)   For example, suppose that an investor owns a passthrough in which the remaining mortgage balance at the beginning of some month is $290 million. Assuming that the SMM is 0.5143% and the scheduled princi- pal payment is $3 million, the estimated prepayment for the month is:   0.005143 ´ ($290,000,000 - $3,000,000) = $1,476,041   PSA Prepayment Benchmark The Public Securities Association (PSA) prepayment benchmark is expressed as a monthly series of CPRs. The PSA benchmark assumes that prepayment rates are low for newly originated mortgages and then will speed up as the mortgages become seasoned. The PSA prepayment benchmark assumes the following prepayment rates for 30-year mortgages: (1) a CPR of 0.2% for the first month, increased by 0.2% per year per month for the next 30 months when it reaches 6% per year, and (2) a 6% CPR for the remaining years. This benchmark is referred to as "100% PSA" or simply "100 PSA." Slower or faster speeds are then referred to as some percentage of 100 PSA. For example, 50 PSA means one-half the CPR of the PSA benchmark prepayment rate; 150 PSA means 1.5 times the CPR of the PSA benchmark prepayment rate;     300 PSA means three times the CPR of the benchmark prepayment rate. A prepayment rate of 0 PSA means that no prepayments are assumed. It is important to understand that the PSA benchmark is commonly referred to as a prepayment model, suggesting that it can be used to esti- mate prepayments. Characterization of this benchmark as a prepayment model is incorrect. It is simply a market convention describing what the PSA believes the pattern will be for prepayments. It is worthwhile to see a monthly cash flow for a hypothetical passthrough given a PSA assumption since we can use the information in our discussion of collateralized mortgage obligations in the next sec-