Definition of 'Prepayment Model'
A model used to estimate the level of prepayments on a loan portfolio that will occur in a set period of time, given possible changes in interest rates. Prepayment models are based on mathematical equations and usually involve the analysis of historical prepayment trends. Prepayment models are generally used to value mortgage pools such as GNMA securities or other securitized debt products.
As interest rates rise, prepayment models factor in fewer prepayments. If interest rates fall, the opposite effect is accounted for, as more people will refinance their loans.
Investopedia explains 'Prepayment Model'
One of the most notable prepayment models is the PSA Prepayment Model by the Securities Industry and Financial Markets Association. The PSA model assumes increasing prepayment rates for the first 30 months and then constant prepayment rates afterward.
The standard model, referred to as 100% PSA or 100 PSA, assumes that prepayment rates will increase by 0.2% for the first 30 months until they peak at 6% in month 30. 150% PSA would assume 0.3% (1.5 x 0.2%) increases to a peak of 9%, and 200% PSA would assume 0.4% (2 x 0.2%) increases to a peak of a 12% prepayment rate.