Skip navigation

This series of blogs is derived form my discussions on the LinkedIn Quant Finance: What is the best approach to handling CMBS &/or RMBS Credit Risk analysis? discussion forum.

I agree with prepayments in RMBS space but I was looking for more than the usual accepted, standard knowledge. Sort of industry wisdom versus textbook stuff. Argyn had suggested other loss models and Russell had explained how migrating FICO scores affect RMBS portfolios. The discussion continues:

For example if Argyn had not mentioned bond pricing models that would not have jogged my memory about the negative correlations between RMBS and CMBS. This negative correlation is something anybody can test for themselves. I don’t have the data or the models so it is from memory. I would suggest testing residential losses against commercial losses by MSAs. There are some economic lag effects but never got to complete this study.

This negative correlation could be interpreted as consumer spending lags job loss or growth. Therefore, if the recession is long enough we can expect an increase in commercial property losses even as residential properties begin to recover. If the recession is short you won’t notice this lag because most consumers and property owners have some staying power. If the recession is too long then both are negatively affected.

OK let me step out of the ‘conventional space’ to make this discussion more interesting and hope that others would join in.

My thinking was once you have bought a deal, it is yours good, bad and ugly. So accounting for prepayments (the bad) in RMBS is a poor strategy. As a tactic you need to do it but as a strategy it is poor. And you are always stuck with the economy, and whatever it does.

My strategy for both RMBS & CMBS was to determine the CVaR (the ugly) that an investor was willing to tolerate within some investment horizon, say 5 years from the underwritten parameters.

To do this you need to know how a deal would perform over this investment horizon, from the underwritten parameters. That means prepayments are (1) of little value as they are driven by future changing economic fundamentals, and (2) they are management tools to deal with the bad.

The question I then had to answer was how do we minimize the ugly (CVaR) given only underwritten parameters?

Before I ran into the RMBS incomplete data problem and abandoned RMBS, I tested FICO scores. Could we use underwritten FICO scores to predict future losses in RMBS space? Once you have bought the deal you can watch credit scores deteriorate, but we want to avoid that as much as we can.

One of the things I did was to analyze the distribution of FICO scores in the ‘universe’ against the distribution in the defaulted assets. And to my surprise there was no difference. Underwritten FICO scores are unable to predict future deterioration of individual credit worthiness. That is one of the main reasons we abandoned RMBS because with incomplete data everything hinges on FICO scores. Again you can test this yourself.

I believe that from a forecasting perspective, FICO scores are a proxy for either disposal or discretionary income, that the two are highly correlated – I haven’t done this study, it is a guess having looked at tons of data.

In CMBS one can build a model to forecast future losses at a deal or portfolio level, and then determine from the CVaR-tolerance and investment-horizon, which portfolio you would invest in.

Anyone else seen these types of problems with FICOs?

Anybody tried the investment strategy I’ve outlined above?


Discalimer: This blog is purely for informational/educational purposes and is not intended to either negate or advocate any product, service, political position or persons.
Creative Commons License
QuantumRisk Blog Posts by Benjamin T Solomon is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.
Based on a work at


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: