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Preliminary; not for quotation. NBER working papers are distributed informally and in limited number for comments only. They should not be quoted without written permission of the author. This report has not undergone the review accorded official NBER publications; in particular, it has not yet been submitted for apprbva1 by the Board of Directors. *This research was supported by the National Bureau of Economic Research. Thanks are due to Sean Becketti for excellent research assistance and to Lung Fei Lee for advice on statistical issues. The order of the authors names was selected by a random device.
Willis et al. (Mon,) studied this question.
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