Mortgages and MBS

8 important questions on Mortgages and MBS

What about mortgage loans?

Fixed, variable, residential or commercial purposes (we focus on fixed residential). Agency or conforming loans are eligible to be securitised. Government owned agencies are generally for low-principal and more creditworthy loans.

Non-agency, non-conforming loans belong to private label. Alt-A or even Subprime. In which the later may involve prior deliquency. A lot of these mortgages are ARMs, adjustable-rate mortgages. These usually have low initial, teaser, rates which increase after a few years.

WHat about a prepayment option?

Redeem principal at any time and be freed of the obligation to make further payments. Valuable when rates have fallen. Basically it's like an American call option. Strike is just the principal outstanding, vs. the PV of the cash flows.

What about mortgage-backed securities?

Secondary market activity, creating systemic risk. Mortgage loans are combined in pools. Mortgage pass-through, the cash flows from the underlying mortgages are passed from the borrowers to the investors with some short processing delay. Mortgage services manage the below in exchange for a fee. Sometimes the pools are guaranteed. Recourse is more difficult in America.
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What about a mortgage pool?

- WAC, weighted-average coupon (coupon is lower than original-WAC due to fee)
- WAM, weighted-average maturity
- Factor is the ratio of the current to the original principal amount outstanding. Can be used to measure prepayments.
Coupon and age are most important for pricing!

How about prepayments in pools?

Curtail, partially prepayment. For purpose of valuation, measure the principal amount prepaying as a percentage of the total principal outstanding. Single Monthly Mortality rate (SMM). Annualizing creates a constant or conditional prepayment rate (CPR).

1 - (1-SMM)^12

WHat about specific pools and TBAs?

For specified pools the price of a trade reflects the characteristics of the particular pool. High-loan balance pools will trade for relatively low-prices, due to higher expected prepayments.

TBA = to be announced. TBA seller will pick the cheapest-to-deliver (CTD) pool, that is, the pool that is worth the least subject to the issuer, maturity, and coupon requirements. Pay-up, extra value above TBA. Usually able to have a below par price.

What about MBS valuation and trading?

Using Monte-Carlo vs. using a tree. Difference is that the tree assumes independence, while a burnout would be dependent. MC helps on dependence but drawbacks are: computationally complex and impossible to price Bermudan.

Can use empirical hedge-ratios. Higher coupons prepay faster, thereby more like shorter-term securities meaning lower interest rate sensitivity.

OAS, option-adjusted spread. Relative value measure for MBS. Blend of relative and left-out factors. Rely on mean reversion. Usually a relative combined with a supporting story can be convincing.

WHat about price-behavior of MBS?

If no prepayments, the positively convex. Higher CPR has lower BPV (no positive convexity). It is NEGATIVELY convex for lower rates (due to at par, called). Interest-only pool can have negative BPV or duration which is quite unusual.

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