During the 58 years for which Treasury yield data is available, the monthly treasury average has never been as low as it was last month.
December ended with the MTA at 0.18167 percent. Based on an analysis of Federal Reserve data back to 1953, the index has never been this low.
The MTA eased from 0.19583 percent calculated for November.
In December 2010, the index was much higher at 0.31833 percent.
The MTA was calculated using the daily average for the one-year Treasury note yield during each of the past 12 months as reported by the Federal Reserve. During December, the one-year yield averaged 0.12 percent, up from 0.11 percent in November.
While the MTA is used as an index on some adjustable-rate mortgages, a much more broadly used ARM index is the yield on the one-year Treasury note. According to the Department of the Treasury, the one-year Treasury yielded 0.12 percent as of the end of December, the same as the end of November.
The one-year Treasury yield closed Tuesday also at 0.12 percent.
ARM share of overall new activity was 5.06 percent in the U.S. Mortgage Market Index report from Mortech Inc. and MortgageDaily.com for the week ended Dec. 30. A week earlier, the share was 5.33 percent.