By using a variable-rate annuity, Messrs Feldstein and Samwick can assume higher average returns (and hence lower benefit cuts) than the Democratic economists.

ECONOMIST: Social Security reform: The battle of the boffins | The

The same is true of the assumption that the mortgage repayment will rise (it could be a longer-term fixed rate for example), or by how much - we have used the Standard Variable Rate (SVR), but the borrower may be using a mortgage which charges a different interest margin.

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