Cash-Flow Modeling and OAS in Mortgage-Backed Security Valuation
Summary
The document explains the basic valuation framework for residential and commercial mortgage-backed securities: estimate the security’s cash flows and discount them to calculate present value. The central difficulty is forecasting those cash flows, especially mortgage prepayments, defaults, and loss severity. Prepayment behavior depends on interest rates, so an option-adjusted spread approach is presented as more appropriate than relying on a static prepayment assumption for securities with embedded options.
The answers also outline how structured products can be valued by decomposing their components, accounting for asset correlations and contractual covenants, and tracing how those features affect cash-flow timing and allocation. Valuation methods are described as broadly consistent for agency mortgage securities, while private pools may require more tailored treatment and collateralized mortgage obligations add tranche complexity. The document gives a framework rather than a specific model or calibration procedure; its estimates depend on assumptions about loan characteristics, prepayment and credit behavior, forward rates, and discount spreads.
Key ideas
- MBS valuation discounts projected security cash flows to present value.
- Prepayments, defaults, and loss severity are major sources of cash-flow uncertainty.
- Interest-rate-sensitive prepayments make option-adjusted spread analysis useful for securities with embedded options.
- Structured-product valuation must account for component assets, correlations, covenants, and cash-flow sequencing.
- Private pools and tranche structures can make valuation more complex than for agency MBS.
Tags
Full text
# How are prices calculated for commercial/residential mortgage-backed securities? # How are prices calculated for commercial/residential mortgage-backed securities? What is the theoretical/mathematical basis for the valuation of [C]MBS and other structured finance products? Is the methodology mostly consistent across different products? ## Answer by Joshua Ulrich (score 6, accepted) https://quant.stackexchange.com/a/444 I can't speak for all structured products but valuing a MBS is straight-forward, but not easy. It's straight-forward because you just need to calculate the net present value of the discounted cash flows. That said, accurately determining those cash flows is hard. The most difficult cash flows to determine--prepayments and defaults/severity--also have the largest impact on MBS value. Prepayments are highly dependent on interest rates, so an OAS (option-adjusted spread) approach is superior to a static assumption. The valuation is fairly consistent across different MBS, especially for MBS from the GSEs. A private MBS would likely be more complicated because they don't have the pooling restrictions of the GSEs. CMOs are even more complicated to value because of the tranche structure. ## Answer by glyphard (score 2) https://quant.stackexchange.com/a/340 The framework for valuing structured finance products in general is based on the nature of the cashflows in the product. - Decompose the constituent components of the structure. - Make some choices about handling the correlations between assets in the structure. - Review the covenants of the structure and their impact on the cashflows (sequence of events). - Compute. ## Answer by Hypergravity (score 2) https://quant.stackexchange.com/a/2417 Mortgage backed securities are valued by calculating the net present value (NPV) of cash flows they are expected to generate. These cash flows are predicted using a model that incorporates all the contractual characteristics of the security and the underlying loans, as well as assumptions on things like prepayment speed, default speed, loss severity, and forward interest rates. Discount rates are generally determined by adding a spread to the yields on the pricing yield curve. This spread is an observed or assumed market option adjusted spread (OAS) in the case of securities with embedded options or Z-spread in the case of option free bonds.
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