Then there's the unknown variable of an anticipated increase in turnout in the African-American community.

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The unknown variable in the AUD scenario is China.

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Traveling 120 miles an hour on a road built for cars can be perilous enough, but when you're -- almost literally -- heading into the unknown, it's a variable you can probably do without.

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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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