Welcome to The S-Curve
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The name of the blog, the S-Curve, is a reflection of our logo and the central feature of our prepayment model. S-curves are seen in nature in many phenomenon, from population growth to prepayment and default models. Our first S-curve, in the early 1990s, used the arctangent function, then piece-wise linear functions, and evolved over time to be more complex and vary by FICO, loan size and LTV. This evolution encapsulates both the timeless nature of fundamental relationships and constant innovation to describe them better over time.
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FRED Adds AD&Co’s GSE-and-Borrower-Option-Adjusted Spreads for CRT IndicesNewsAD&Co US Mortgage High Yield Indices
The Federal Reserve Economic Data (FRED) portal, housed by the Federal Reserve Bank of St. Louis, has been publishing AD&Co’s CRT indices since 2019. These series posted under the overall name of “US Mortgage High-Yield” include total return rates and credit and option-adjusted spreads (crOAS) – a projected return’s spread over Treasury (in the past, Libor). These series are available going back to 2014-end and tiered by CRT initial supports.
Tier 0 includes all CRTs with under-25 bps support; Tier 1 bonds have supports exceeding 25 bps, but not 95 bps; Tier 2 has support from 95 bps to 175 bps; Tier 3 – from 175 bps to 375 bps, and, finally, Tier 4 – above 375 bps. The actual bond’s name (As, Ms, or Bs) that matches each tier can vary over time and between Fannie Mae’s CAS and Freddie Mac’s STACR transactions. We define Mid-Tier as the aggregation of Tiers 1 through 3. A CRT to be included in an index must have a factor of 0.25 or higher.
While actual rates of investment return are computed model-free, crOAS levels come from the AD&Co model. Importantly, the crOAS indices that date back to 2014 do not account for the GSE call option embedded in a CRT and therefore overstate the expected return. See, for example, index CROASMIDTIER for Mid-Tier or index CROASTIER0 for Tier 0; the latter currently shows crOAS of about 600 bps.
What is New?
Over the last couple of years, AD&Co developed a model to account for embedded GSE calls. Most CRTs are now issued with a five-year call and a cleanup call. Exercised in the interest of the GSEs, those options reduce investor return. Our December 2024 Quantitative Perspectives[1] laid out the theoretical foundation of our methods. A subsequent September 2025 Pipeline article[2] listed results of the production analysis across the entire CRT cash market.
We have been using the new method in our CRT Monitor monthly publication for the last several months. We have also started sending the new series to FRED, which has adopted it with an announcement. The new crOAS series goes back only to June 30, 2025, and is reported by the same tiers as the previously computed ones. To indicate the difference in the series, the new series contains “GSE and Borrower Options-Adjusted Spread” in the names. A screenshot of FRED’s onboarding, showing all the indices together, is seen below.
Source: FRED As expected, the more protected CRTs are priced at tighter, more realistic, crOAS levels. They never reach many hundreds of basis points when the GSE option is accounted for.
[1] A. Levin and N. Salwen, Valuation of Credit Risk Transfer with Embedded Calls, Quantitative Perspectives, Dec 2024.
[2] A. Levin, Comparative Valuation of CRTs with and without Embedded GSE Calls, Pipeline 191, Sep 2025.
The S-Curve Archives
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Products
Today marks the publication of Chris Widman's Quantitative Perspective, a comprehensive article on the newest member of our LoanDynamics suite, the Auto LoanDynamics Model. Auto LDM will be integrated into vendor systems and AD&Co tools, allowing users to perform analysis on auto loan and ABS positions.
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EventsSince 1970, April 22nd has been the annual day to appreciate our planet and recognize the importance of protecting it. But more and more, we realize that everyday needs to be Earth Day, and that we need to take better care of the place that gives us life.
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Thoughts
To seek "causes" of poverty in this way is to enter an intellectual dead end because poverty has no causes. Only prosperity has causes. – Jane Jacobs, Activist and Author
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Events
CRTcast, a new podcast series under Freddie Mac’s Home Starts Here programming, focuses on credit risk transfer (CRT) and it’s three spokes: securities, (re)insurance and mortgage insurance. Freddie Mac leadership together with CRT industry experts cover current and relevant topics.
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News
We are proud to announce that Richard Cooperstein has accepted the position of co-chair of the Structured Finance Association’s (SFA) Regulatory Capital & Liquidity committee.
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NewsToday we acknowledge the Year of the Ox. Happy Lunar New Year! We stand in solidarity with the Asian community against all violence and racism. Here’s to a year of peace, health and prosperity.
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NewsThis February, AD&Co celebrates a central part of American History—Black History. The richness of the contributions of the Black community as a whole, and innumerable remarkable individuals, can not be overstated.
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Thoughts
The January 14, 2021 revisions of the Preferred Stock Purchase Agreements between the Treasury and the GSEs[i] (Government Sponsored Enterprises) along with the Treasury Department Blueprint on Next Steps for GSE[ii] Reform perhaps represent the end of a decade- long effort to create multiple competitive enterprises and end the government support of the GSEs.
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NewsMartin Luther King, Jr. was a great leader and inspirational speaker. His wisdom can serve as a guide for as long as we remember him. Andrew Davidson & Co would like to acknowledge a fraction of what he gave us with two relevant quotes that seem fitting in 2021.
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Thoughts
In the spring of 2019, National Association of Realtors® (NAR), together with financial-market experts Susan Wachter (Wharton) and Richard Cooperstein (Andrew Davidson & Co., Inc.) proposed completing the transition of Fannie Mae and Freddie Mac (Enterprises) into market utilities in a publication entitled “A Vision for Enduring Housing Finance Reform.” This work builds on Richard Cooperstein and Andrew Davidson’s 2017 paper