C3-5-03, Calculating the Weighted-Average Pool Accrual Rates for ARM Flex Pools Using a Weighted-Average MBS Margin (04/01/2009)

In the following example, assume that the lender wants to place the following three ARM Plan 57 ARMs into a weighted-average ARM Flex MBS pool with a standard remittance cycle. All of the mortgages in the pool will be serviced under the special servicing option and will have a guaranty fee of 0.35%. All of the mortgages have borrower-purchased mortgage insurance.

To develop a weighted-average MBS margin, the lender must reduce the mortgage margin for each mortgage to be included in the pool by the applicable guaranty fee percentage and then by the desired fixed servicing fee (and, if applicable, by the periodic renewal premium for lender-purchased mortgage insurance) to arrive at a loan-level MBS margin. The difference between the MBS margins for the mortgages in the pool will be exactly equal to the differences in their mortgage margins. The weighted-average pool accrual rate is then determined by first reducing each individual mortgage interest rate by the servicing spread for the mortgage (the sum of the guaranty fee and the desired servicing fee and, if applicable, the periodic renewal premium for lender-purchased mortgage insurance) and then developing a weighted-average of the net mortgage interest rates. This same procedure also is used to establish the maximum weighted-average pool accrual rate (and any applicable minimum weighted-average pool accrual rate), using the weighted-average of the net mortgage interest rate ceilings (or floors) of the mortgages in the pool.

Step One:

Determine the loan-level MBS margin, using a 0.250% standard servicing fee.

(This step is not necessary. It is included for informational purposes only.)

Step Two:

Determine the net mortgage interest rate.

Step Three:

Determine the weighted-average pool accrual rate.

15,970/180,000 = 8.872%, rounded to three decimal places.

Step Four:

Determine the net mortgage interest rate ceiling.

Step Five:

Determine the maximum weighted-average pool accrual rate.

26,770.00/180,000 = 14.872%, rounded to three decimal places.

Step Six:

Determine the minimum weighted-average pool accrual rate (if the mortgages have an interest rate floor). Since the mortgages in this example do not have an interest rate floor, this step is not necessary. It is shown for illustration purposes only.

First, find the net mortgage interest rate floor by following Step Four, substituting the mortgage interest rate floor for the ceiling.

Then, follow Step Five to find the minimum weighted-average pool accrual rate, using the net mortgage interest rate floor just calculated for each mortgage instead of the mortgage interest rate ceilings.