You can calculate a stream of earnings off the asset, apply an assumed discount rate and determine a present value for the assets.

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Using ACNielsen data from 61, 500 households, Hausman and Leibtag calculate that grocery shoppers are 20% better off--not the full 27%--with a superstore shopping trip.

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The system was always meant to include annual payments or subsidies as well as a one-off installation payment, but the government had not yet worked out how to calculate them, he said.

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Attorneys will also likely square off over how much oil gushed into the gulf, another key figure that will be used to calculate how much money BP might owe.

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