Short-sellers normally have to locate shares to borrow before they go ahead and sell them, hoping they can buy them back later at a discount and pocket the difference.

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The company was seeing how much of a discount was needed to make a significant difference to the number of DVDs it sells.

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The shares have climbed more than 35% since, yet still trade at a discount of 23% to book value, the difference, or net worth, between its assets and its liabilities.

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The big question is: Will shoppers notice a difference when they hit the malls and discount stores on so-called Black Friday -- the day after Thanksgiving, when retailers traditionally "go into the black" -- and during the holidays?

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Steve Hodge, treasurer of Shell, an Anglo-Dutch oil giant, says that a difference of only one percentage point in the discount rate can change the value of, say, an oil refinery with a 15-year life by hundreds of millions of dollars.

ECONOMIST: Corporate finance