Cost of savings index - COSI

Cost of savings index - COSI

Cost of savings index, or COSI, is a weighted average of interest rates paid on savings accounts. Originally, COSI was based on the rates paid by subsidiaries of Golden West Financial Corporation (GDW) that operated as World Savings. The index was used as a benchmark rate for adjustable-rate mortgages (ARMs). Since World Savings merged with Wachovia, however, the original COSI is still published but no longer used for new mortgages. Wachovia publishes a similar monthly index, called the W-COSI, which is used for new ARMs.