DEFINITION of 'Cost Of Savings Index - COSI Index'

A popular index used for certain adjustable-rate mortgages (ARMs). The index is a weighted average of the rates of interest on the deposit accounts (sometimes called cost of savings) of the federally insured depository institution subsidiaries of Golden West Financial Corporation (GDW). These subsidiaries currently operate under the name World Savings.

GDW computes the COSI on the last day of each calendar month and announces it on or near the last business day prior to the fifteenth day of the following calendar month.

BREAKING DOWN 'Cost Of Savings Index - COSI Index'

The COSI index is a stable index. Historically, it is not as volatile as some other popular mortgage indexes, such as the one-month LIBOR index. It also tends to lag other mortgage indexes when it comes to adjusting to changes in interest rates.

  1. Current Index Value

    The most recently published value of an underlying interest rate ...
  2. Mortgage Index

    The benchmark interest rate an adjustable-rate mortgage's fully ...
  3. ARM Index

    The benchmark interest rate to which an adjustable rate mortgage ...
  4. Index

    An index measures the performance of a basket of securities intended ...
  5. Fully Indexed Interest Rate

    A fully indexed interest rate is a variable interest rate that ...
  6. Teaser Rate

    An initial rate on an adjustable-rate mortgage (ARM). This rate ...
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