Market convention for expressing mortgage prepayment speed relative to a standard benchmark.
The PSA prepayment model is a market convention for expressing mortgage prepayment speed relative to a standard benchmark.
The PSA model matters because mortgage investors need a common shorthand for comparing prepayment assumptions. Instead of describing every pool from scratch, analysts can say a pool is expected to prepay faster or slower than a benchmark speed.
For borrowers, the term is useful because it connects ordinary refinance and payoff behavior to the market assumptions behind mortgage-backed securities. Borrowers do not choose a PSA speed, but their collective behavior helps determine whether real prepayments are faster or slower than expected.
Borrowers usually do not encounter PSA language in consumer mortgage documents. It appears in MBS analytics, prepayment modeling, and investor discussion.
The term becomes practical when explaining how analysts compare different mortgage pools and how prepayment expectations affect Average Life, Prepayment Risk, and MBS pricing.
| PSA wording | Plain-language idea |
|---|---|
| Slower than benchmark | Borrowers are expected to prepay less quickly |
| Benchmark speed | Prepayments are modeled at a standard reference pace |
| Faster than benchmark | Borrowers are expected to prepay more quickly |
An analyst comparing two MBS pools may describe one as having a faster assumed PSA speed because the loans are expected to refinance or pay off more quickly. That assumption can shorten expected average life and change the way investors value the cash flow.
The PSA model differs from Conditional Prepayment Rate because CPR is a prepayment-speed measure, while PSA is a benchmark convention for comparing assumed speeds.
It also differs from Single Monthly Mortality. SMM is a monthly prepayment measure. PSA is a broader market convention used to express prepayment assumptions.
It also differs from Prepayment Risk. Prepayment risk is the exposure to unexpected early payoff. The PSA model is one way analysts communicate prepayment assumptions.