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ARM Indexes: MTA (12-MAT)

Moving Average of the Monthly values of the One-year
Treasury Constant Maturity, 1990s (explanation)

Source: Federal Reserve (raw data)

Year Value
Jan-91 7.8825
Feb-91 7.7758
Mar-91 7.6225
Apr-91 7.4600
May-91 7.2800
Jun-91 7.0975
Jul-91 6.9525
Aug-91 6.8167
Sep-91 6.6500
Oct-91 6.4675
Nov-91 6.2825
Dec-91 6.0808
Year Value
Jan-92 5.8583
Feb-92 5.6508
Mar-92 5.4858
Apr-92 5.3383
May-92 5.1767
Jun-92 5.0150
Jul-92 4.8325
Aug-92 4.6067
Sep-92 4.4142
Oct-92 4.2150
Nov-92 4.0458
Dec-92 3.9450
Year Value
Jan-93 5.8583
Feb-93 5.6508
Mar-93 5.4858
Apr-93 5.3383
May-93 5.1767
Jun-93 5.0150
Jul-93 4.8325
Aug-93 4.6067
Sep-93 4.4142
Oct-93 4.2150
Nov-93 4.0458
Dec-93 3.9450
Year Value
Jan-94 3.4342
Feb-94 3.4375
Mar-94 3.4775
Apr-94 3.5600
May-94 3.6917
Jun-94 3.8542
Jul-94 3.9983
Aug-94 4.1658
Sep-94 4.3425
Oct-94 4.5425
Nov-94 4.7692
Dec-94 5.0158
Year Value
Jan-95 5.3100
Feb-95 5.6025
Mar-95 5.8383
Apr-95 6.0142
May-95 6.1350
Jun-95 6.1925
Jul-95 6.2233
Aug-95 6.2325
Sep-95 6.2483
Oct-95 6.2367
Nov-95 6.1933
Dec-95 6.1008
Year Value
Jan-96 5.9483
Feb-96 5.7850
Mar-96 5.6383
Apr-96 5.5475
May-96 5.4867
Jun-96 5.4567
Jul-96 5.4708
Aug-96 5.4925
Sep-96 5.4858
Oct-96 5.5033
Nov-96 5.5000
Dec-96 5.4992
Year Value
Jan-97 5.5125
Feb-97 5.5558
Mar-97 5.6050
Apr-97 5.6433
May-97 5.6808
Jun-97 5.7000
Jul-97 5.6900
Aug-97 5.6642
Sep-97 5.6550
Oct-97 5.6292
Nov-97 5.6217
Dec-97 5.6250
Year Value
Jan-98 5.6300
Feb-98 5.5992
Mar-98 5.5808
Apr-98 5.5467
May-98 5.4958
Jun-98 5.4600
Jul-98 5.4367
Aug-98 5.4217
Sep-98 5.3925
Oct-98 5.3250
Nov-98 5.2133
Dec-98 5.1358
Year Value
Jan-99 5.0517
Feb-99 4.9908
Mar-99 4.9400
Apr-99 4.8892
May-99 4.8317
Jun-99 4.7825
Jul-99 4.7567
Aug-99 4.7292
Sep-99 4.7283
Oct-99 4.7733
Nov-99 4.8825
Dec-99 4.9675


Explanation of this index

MTA (aka 12-MAT) is an index used to govern changes in certain Adjustable Rate Mortgages (ARMs), notably Option and FlexPay-style ARMs which feature monthly adjustment periods. MTA stands for "Moving Treasury Average".

The MTA, sometimes called MAT or 12-MAT, is a "derived" ARM index. It is produced by adding together other published index values and dividing the sum by the number of entries to produce a final single value.

To produce the monthly value for the MAT, a lender will add the last twelve monthly values of the one-year US Treasury Constant Maturity (not Treasury "bill") and divide the total by 12 (entries). The result is the new index value.

To that value, the lender will add a markup, called a "margin", and the sum of the two becomes your loan's new interest rate. For Option and FlexPay ARMs, the margin is typically 250 basis points (2.5%). Different lenders may use larger or smaller margins.

Since a "moving average" series could use any number of Treasury values, some lenders identify the MTA as 12-MAT (their own shorthand for 12-month moving average of the one-year Treasury).

Because it is a moving average, the MTA isn't as volatile as the index it is derived from (see this chart for a graphic demonstration). This can work to your advantage when interest rates are rising, but when rates are falling, the moving average will prevent your mortgage rate from falling rapidly.

Indexes comparable to the MTA include the 11th District Cost of Funds (COFI) and COSI, the cost-of-savings index produced by Golden West Financial (World Savings).

The latest value of this index is posted in the Current Indexes table.

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