The standard model (also called "100% PSA") works as follows: Starting with an annualized prepayment rate of 0.2% in month 1, the rate increases by 0.2% each month, until it reaches 6% in month 30. From the 30th month onward, the model assumes an annualized prepayment rate of 6% of the remaining balance. Each monthly prepayment is assumed to represent full payoff of individual loans, rather than a partial prepayment that leaves a loan with a reduced principal balance.
Variations of the model are expressed in percent; e.g., "150% PSA" means a monthly increase of 0.3% in the annualized prepayment rate, until the peak of 9% is reached after 30 months. The months thereafter have a constant annualized prepayment rate of 9%.
1667% PSA is roughly equivalent to 100% prepayment rate in month 30 or later.