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NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competencies and with regard for appropriate balance.

This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

International Standard Book Number 0-309-05737-X

Library of Congress Catalog Card Number 91-28051

Cover photograph: LES MOORE/UNIPHOTO

Copyright 1997 by the National Academy of Sciences. All rights reserved.

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