hard · Asset-Backed Securities

A modeler builds a monthly cash-flow engine for a fixed-rate ABS and applies prepayments and defaults in the following per-period order: (1) accrue interest on the beginning balance, (2) apply scheduled principal, (3) apply the SMM voluntary prepayment to the post-scheduled balance, (4) apply the MDR (monthly default rate) to the balance remaining after prepayment. A peer modeler uses the identical SMM and MDR but applies defaults *before* voluntary prepayments (swapping steps 3 and 4).

For a pool with positive SMM and MDR, how do the two engines' projected *voluntary prepayment* dollars compare in a given month, and why?

  1. Identical voluntary prepayment dollars, because SMM and MDR are applied to the same beginning balance and the ordering of two independent percentage hazards is commutative within a month.
  2. Lower voluntary prepayments in the defaults-first engine, because defaults remove balance before the SMM is applied, shrinking the base to which the voluntary prepayment rate attaches.
  3. Higher voluntary prepayments in the defaults-first engine, because removing defaults first leaves a cleaner performing balance that the SMM is grossed up against to preserve the target speed.
  4. Identical voluntary prepayments but different defaults, because the SMM base is fixed by the scheduled balance while only the MDR base shifts with ordering.

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