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CFA Level I · CFA Level I Exam

Mortgage-Backed Security (MBS) Instrument and Market Features

A mortgage-backed security (MBS) is a bond whose cash flows come from a pool of mortgage loans. Investors receive pooled interest, scheduled principal and prepayments. To solve questions, identify the security type, find who bears prepayment or credit risk, then judge how rate changes affect cash flows, average life and value.

What this chapter covers

This chapter covers securitized mortgage products. You start with the mortgage loan itself: its payment structure, interest rate type, and whether the lender can recover only the property (non-recourse) or also pursue the borrower (recourse). You then see how loans are pooled into pass-through securities, and how the pool's cash flows are reshaped into tranches in collateralized mortgage obligations (CMOs).

The central idea is prepayment risk. Borrowers can repay early, usually when rates fall, so investors get principal back when reinvestment yields are low. This is contraction risk. When rates rise, prepayments slow and the security lasts longer than expected. This is extension risk. Measures such as the single monthly mortality rate (SMM), conditional prepayment rate (CPR) and the PSA benchmark let you describe and compare prepayment speeds. Later topics show how CMO structures redistribute this risk, how non-agency RMBS use credit enhancement to manage default risk, and how CMBS differ because commercial loans are underwritten on property cash flow and often have call protection.

The chapter connects to the rest of Fixed Income. It builds on bond pricing, duration and convexity, since MBS show negative convexity from the borrower's prepayment option. It also links to credit analysis, because non-agency and commercial deals rely on tranching and subordination. Beyond that, it ties to Derivatives, as the prepayment option behaves like an embedded call option.

Fixed Income carries a meaningful share of the Level I exam, and securitization questions are usually conceptual with light arithmetic, so they are very winnable with three-option questions and no penalty for wrong answers. The same logic recurs: who holds the option, who takes the risk, and what happens when rates move. If you understand it once, you can eliminate two wrong options quickly across pass-throughs, CMOs and CMBS, and the ideas also strengthen your duration, convexity and credit answers elsewhere.

Mortgage-Backed Security (MBS) Instrument and Market Features: topics in the order to study them

  1. 1Mortgage Loan Basics and TypesEvery later product is built from loans, so you need payment structure, rate types and recourse first.
  2. 2Mortgage Pass-Through SecuritiesThis shows how loans are pooled and how cash flows reach investors, before any restructuring.
  3. 3Prepayment Risk and MeasurementContraction and extension risk, SMM, CPR and PSA explain the behaviour of pass-throughs and set up CMOs.
  4. 4Collateralized Mortgage Obligations (CMOs)CMOs reallocate prepayment risk across tranches, so you must understand the risk first.
  5. 5Non-Agency RMBS and Credit EnhancementOnce prepayment is clear, add credit risk and the tools that manage it, such as subordination.
  6. 6Commercial Mortgage-Backed Securities (CMBS)CMBS reverse the emphasis from prepayment to credit and call protection, so they are easier to learn last as a contrast.

How to prepare Mortgage-Backed Security (MBS) Instrument and Market Features

Aim to understand the cause and effect first, then practise the few formulas. Short daily sessions work well on a phone.

  1. Read the mortgage loan basics and write one line each for fixed-rate, adjustable-rate, recourse and non-recourse loans.
  2. Draw a simple flow: borrowers, servicer, pool, investors. Mark where interest, scheduled principal and prepayments appear.
  3. Learn the prepayment measures. SMM is the share of the remaining balance prepaid in a month, and CPR annualizes it: CPR = 1 − (1 − SMM)^12. Practise converting both ways on your calculator, using the y^x key on the TI BA II Plus.
  4. Build a rate-up and rate-down table: for each scenario note prepayments, average life, price and the risk (extension or contraction).
  5. For CMOs, list each tranche type and state who gains protection and who absorbs the risk. Do the same for subordination in non-agency RMBS.
  6. Compare CMBS with residential MBS in a two-column list covering borrower, underwriting, call protection and prepayment.
  7. Finish with timed three-option questions at about 90 seconds each and review every miss by the risk it tested.

Common mistakes in Mortgage-Backed Security (MBS) Instrument and Market Features

  • Saying falling rates always help MBS holders.

    Fix: Remember the prepayment option. Falling rates speed prepayments and cap price gains, which is negative convexity.

  • Mixing up contraction and extension risk.

    Fix: Link them to rates: falling rates mean faster prepayments and a shorter life (contraction); rising rates mean slower prepayments and a longer life (extension).

  • Confusing SMM and CPR, or forgetting to annualize correctly.

    Fix: Use CPR = 1 − (1 − SMM)^12 and check that CPR is the larger number.

  • Believing a CMO removes prepayment risk.

    Fix: Treat CMOs as a way to shift risk between tranches. The total risk of the pool stays the same.

  • Treating agency and non-agency MBS as having the same credit risk.

    Fix: Agency securities carry agency backing, so prepayment is the main concern. Non-agency securities carry credit risk and need credit enhancement.

  • Applying residential prepayment logic to CMBS.

    Fix: For CMBS, focus on property cash flow, call protection and balloon or refinancing risk.

Last-day revision: Mortgage-Backed Security (MBS) Instrument and Market Features

  • An MBS pays investors from a pool of mortgage loans: interest, scheduled principal and prepayments.
  • Recourse loans let the lender claim beyond the property; non-recourse loans limit the lender to the collateral.
  • Falling rates raise prepayments and cause contraction risk; rising rates slow them and cause extension risk.
  • MBS have negative convexity because borrowers hold a prepayment option.
  • CPR = 1 − (1 − SMM)^12, and SMM = 1 − (1 − CPR)^(1/12).
  • A higher PSA speed means faster prepayments.
  • CMOs redistribute prepayment risk across tranches; they do not remove it from the pool.
  • Sequential tranches: earlier tranches get principal first, so they have shorter average lives.
  • Non-agency RMBS rely on credit enhancement such as subordination and overcollateralization.
  • Senior tranches are paid before subordinated tranches, and losses hit the subordinated tranches first.
  • CMBS depend on property cash flow and usually have call protection, so prepayment risk is lower.
  • CMBS loans often have balloon payments, so refinancing risk is important.

Mortgage-Backed Security (MBS) Instrument and Market Features practice questions

Mortgage-Backed Security (MBS) Instrument and Market Features in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Mortgage-Backed Security (MBS) Instrument and Market Features: frequently asked questions

How much calculation is in this chapter?

Very little. The main calculations are SMM, CPR and simple tranche cash-flow reasoning. Expect to spend more of your study time on concepts, such as which tranche bears extension risk, so aim to understand the cause and effect before you practise speed.

What is the difference between a pass-through and a CMO?

A pass-through passes pool cash flows to investors pro rata, so all investors share the same prepayment risk. A CMO splits those cash flows into tranches with different priorities and risk profiles.

Why do MBS have negative convexity?

Borrowers can prepay when rates fall, which limits the price rise of the MBS. When rates rise, prepayments slow and the duration extends. This combination produces negative convexity.

How is CMBS different from residential MBS?

CMBS are backed by loans on income-producing property and are judged on the property's cash flow. They usually have call protection, so prepayment risk is lower, but balloon payments create refinancing risk.