There is a very nice intro at The Third Way on securitization (in the context of The Big Short).  A nice thing they do is link to the SEC filing for a CDO offered by Long Beach Mortgage (that was mentioned in The Big Short).  The filing is more than 200 pages of detail about the bundle of mortgages.  You quickly see why you need Moody’s or some other service to evaluate.  Probably someone has to rewrite the whole “Structured Finance Workstation” using R.

New York, April 30, 2010 — Moody’s Investors Service has downgraded the ratings of 55 tranches from 19 RMBS transactions issued by Long Beach. The collateral backing these deal primarily consists of first-lien subprime residential mortgages. The actions are a result of the continued performance deterioration in Subprime pools in conjunction with home price and unemployment conditions that remain under duress. The actions reflect Moody’s updated loss expectations on subprime pools issued from 2005 to 2007. For details regarding Moody’s approach to estimating losses on subprime pools originated in 2005, 2006, and 2007, please refer to the methodology publication “Subprime RMBS Loss Projection Update: February 2010” available on Moodys.com. To assess the rating implications of the updated loss levels on subprime RMBS, each individual pool was run through a variety of scenarios in the Structured Finance Workstation® (SFW), the cash flow model developed by Moody’s Wall Street Analytics. This individual pool level analysis incorporates performance variances across the different pools and the structural features of the transaction including priorities of payment distribution among the different tranches, average life of the tranches, current balances of the tranches and future cash flows under expected and stressed scenarios. The scenarios include ninety-six different combinations comprising of six loss levels, four loss timing curves and four prepayment curves. The volatility in losses experienced by a tranche due to small increments in losses on the underlying mortgage pool is taken into consideration when assigning ratings. The above mentioned approach “Subprime RMBS Loss Projection Update: February 2010” is adjusted slightly when estimating losses on pools left with a small number of loans. To project losses on pools with fewer than 100 loans, Moody’s first estimates a “baseline” average rate of new delinquencies for the pool (typically 20% for subprime pools). Once the baseline rate is set, further adjustments are made based on 1) the number of loans remaining in the pool and 2) the level of current delinquencies in the pool. The fewer the number of loans remaining in the pool, the higher the volatility and hence the stress applied. Once the loan count in a pool falls below 75, the rate of delinquency is increased by 1% for every loan less than 75. For example, for a pool with 74 loans from the 2005 vintage, the adjusted rate of new delinquency would be 20.20%. If current delinquency levels in a small pool is low, future delinquencies are expected to reflect this trend. To account for that, the rate calculated above is multiplied by a factor ranging from 0.2 to 2.0 for current delinquencies ranging from less than 2.5% to greater than 50% respectively. Delinquencies for subsequent years and ultimate expected losses are projected using the approach described in the methodology publication.

Source: Moody’s downgrades $5.6 Billion of Subprime RMBS issued by Long Beach