With the majority of CDOs being backed by mortgages, the principal-only tranche has a known future cash flow, but at a variable rate of return, while the interest-only tranche has both an unknown cash flow and an unknown rate of return on that cash flow.

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If you look at the data, about 49 percent of homeowners have one measure of vulnerability: high debt payments, high levels of variable interest rate debt, a very high leverage, they own very little of ground homes or they own very little outside of their homes.

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According to some estimates, nearly 90% of loans in the country have a variable interest rate, which means that any rise in the cost of borrowing will impact majority of South Korean households.

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Another issue: more than half of Shurgard's debt is variable-rate, meaning rising interest rates could hurt its profitability, according to Litt.

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The bonds carry a variable interest rate that tracks six-month Euribor rates with an average additional margin of 263 basis points.

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It is a founding member of ERIS, which has developed a futures contract that mimics a variable Libor-based interest rate swap.

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Delinquency rates on variable-interest-rate loans to subprime borrowers, which account for a bit less than 10 percent of all mortgages outstanding, have climbed sharply in recent months.

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Repaying debt has become more difficult in part because loan balances have grown and the interest rates on federal loans have increased as a result of a shift from variable-rate to fixed-rate loans.

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