Moody’s Investors Service has come up with a monitoring approach to evaluating “tail risk” in non-agency MBS that pay scheduled principal and prepayments to the securities on a pro-rata basis and assessing the adequacy of the credit enhancement available to the rated securities. Tail risk is what might be described as the “end of life” risk of a disproportionately large loss (based on current balance of the pool) on the underlying pool at the end of a transaction’s term when few loans remain in the pool and credit enhancements, although high in percentage terms, may be very low in dollar terms. The proposed change in approach at Moody’s will mostly affect...