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Investopedia explains 'ARM Index'
The index to which an adjustable rate mortgage is tied can make a difference over the life of the mortgage. For example, one popular mortgage index is the MTA index. It is a moving average calculation, and therefore has a "lag effect". If interest rates are expected to rise, a mortgage tied to the MTA index might be more economical than a mortgage tied to an index without a moving average calculation, such as the one-month LIBOR index. However, a borrower should consider more than the index when choosing an adjustable rate mortgage. Many other variables, such as the margin and the interest rate cap structure, are important considerations.
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