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Mortgage-Backed Securities 新發展. 姜堯民 政大財管系副教授 于 台科大企業管理研究所 2000/10/26. Financial Intermediation. Traditionally, banks 1) originate a loan; 2) retain the loan in their portfolio of assets, thereby accepting the credit risk associated with the loan;

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mortgage backed securities

Mortgage-Backed Securities新發展





financial intermediation
Financial Intermediation

Traditionally, banks

1) originate a loan;

2) retain the loan in their portfolio of assets, thereby accepting the credit risk associated with the loan;

3) service the loan (collect payments and take legal action if payments were not made; and

4) obtain funds from the public with which to finance their assets.


Recently, banks

1) originate a loan;

2) sell the loan to an investment banking firm that creates a security backed by the pool of loans;

3) the investment bankers obtain credit risk insurance for the pool of loans from an insurance company;

4) the investment banker can sell the right to serve the loan to another bank or a company specializing in servicing loans; and

5) the investment banking firm can sell the securities to individual and institutional investors.

Government National Mortgage Association (GNMA, Ginnie Mae)
  • Federal National Mortgage Association (FNMA, Fannie Mae)
  • Federal Home Loan Mortgage Corporation (FHLMC, Freddie Mac)
mortgage backed securities1
Mortgage-backed securities

- Mortgage passthrough securities, or simply, passthrough

- Collateralized mortgage obligations (CMOs)

- Stripped mortgage-backed securities

Risks faced by mortgage finance intermediaries
  • Credit risk:
      • risk that money lent might not be repaid
  • Cash flow risk:
      • risk that market conditions will alter scheduled cash flows
        • prepayment risk
        • inflation risk
        • exchange risk
        • interest rate risk
  • Liquidity risk:
      • risk that money will be needed before it is due
mortgage passthrough securities
Mortgage Passthrough Securities

- Most popular form of mortgage-backed security

- Represents sale of mortgages

- Taxed only at investor level

- Commits to pay P,I, and prepayments each month as received

- "Modified" pass through guarantees timely payment

- Greatest volume issued by FNMA, FHLMC and GNMA surrogates

- Private entities also issue

- Swap program provided more options to originators

collateralized mortgage obligations
Collateralized Mortgage Obligations

- Debt instrument secured by pool of mortgages

- Appeals to investors who can't tolerate prepayment

- A Derivative: different from the underlying mortgages even though CMO distributions are derived from the mortgages

- Issued in multiple maturities or tranches

- Cash flow is distributed in sequential order (A,B,C,Z tranches)

- A method for issue debt, reduce prepayment risk and shift remaining prepayment risk to the investor

- Quoted with a stated maturity within a range

- Overcollateralized

Collateralized Mortgage Obligations (CMOs): The CMO structure offers issuers a flexible tool with which to design tranches to meet investor needs and respond to market conditions. There are several derivations, including:
  • Sequential pay (SEQ) classes are the most basic classes within a REMIC-based CMO structure.
  • Planned amortization (PACs) classes are designed to produce more stable cash flows by redirecting prepayments from the underlying securities to other classes called support tranches.
  • Targeted amortization (TACs) classes are similar to PACs, but offer investors both call and extension protection.
  • Companion (Support) classes have the most volatile cash flow behavior.
  • Accrual (Z) class investors receive no cash flow from the security until certain other classes are paid off.
  • Interest Only & Principal Only (IO/PO) classes
  • Floating-Rate and Inverse Floating-Rate classes are structured so that the coupon rate payable to the investor adjusts periodically (usually monthly) by adding a certain amount (spread) to a benchmark index.
Prevailing MBS Arbitrage Strategies
  • The primary methodology for valuing mortgage-backed securities is known as Option Adjusted Spread (OAS) analysis. This quantitative technique, coupled with market expertise, enables MBS managers to value fixed income securities with complex embedded options. The application of OAS, which is an implied spread over the risk-less rate for U.S. Treasuries, allows for direct comparison of securities with and without embedded prepayment or call features.
advantages of option adjusted spread analysis
Advantages of Option-Adjusted Spread Analysis
  • OAS analysis gives investors a way to place values on the options inherent in MBSs.
  • OAS methodology analyzes a security over a large number of interest rate paths, provides a summary of almost all possible scenarios, and incorporate the security’s dynamic cash flow into the analysis.
  • Several studies of OAS-based trading strategies for MBSs, like Hayre and Lauterbach (1990), have shown OAS analysis performing well.
disadvantages of option adjusted spread analysis
Disadvantages of Option-Adjusted Spread Analysis
  • As suggested by Babbel and Zenios (1992), OAS is an average return across interest rate paths and maturity. The precision relation between OAS and the interest rate level, and between OAS and maturity are still open questions.
  • Mulvey and Zenios (1994) suspect that OAS in MBSs should decline toward zero with maturity because of less prepayment activity and convergence of price to the par.
How is OAS affected by interest rate changes?
  • How does OAS vary with maturity?
option adjusted spread calculation
Option-Adjusted Spread Calculation

1. generating interest rate paths

CIR model is applied.

2. generating cash flows

Prices and prepayment rates of MBSs of the FNMA are collected from Bloomberg.

interest rate sensitivity
Interest Rate Sensitivity
  • When the initial interest rate level increases, the OAS estimate for ARMs increases, while OAS estimates for other mortgages assets decrease.
  • When the long-term interest rate mean increases, OAS estimates for all mortgage assets decreases.
  • When the mean-reversion parameter, k, increases, OAS estimates for all mortgages assets decreases.
  • When interest rate volatility increases, OAS estimates increases for all types of mortgage assets.
  • A parallel sift of the term structure of interest rates has a negative effect on OAS estimates.
investigating the relationship between oass and maturity
Investigating the Relationship between OASs and maturity
  • Estimating OASs for MBSs with different maturity.
  • Estimating the following model for each type of MBS: