Cause And Correlation In Biology: A User's Guide To Path Analysis, Structural Equations And Causal Inference

This page intentionally left blank Cause and Correlation in Biology A User’s Guide to Path Analysis, Structural Equations and Causal Inference This book goes beyond the truism that ‘correlation does not imply causation’ and explores the logical and methodological relationships between correlation and causation. It presents a series of statistical methods that can test, and potentially discover, cause–effect relationships between variables in situations in which it is not possible to conduct randomised or experimentally controlled experiments. Many of these methods are quite new and most are generally unknown to biologists. In addition to describing how to conduct these statistical tests, the book also puts the methods into historical context and explains when they can and cannot justifiably be used to test or discover causal claims. Written in a conversational style that minimises technical jargon, the book is aimed at practising biologists and advanced students, and assumes only a very basic knowledge of introductory statistics. BILL SHIPLEY teaches plant ecology and biometry in the Department of Biology at the Université de Sherbrooke, Quebec, Canada. His present ecological research concentrates on comparative ecophysiology and the ways in which plant attributes interact to produce ecological outcomes. He has also contributed significantly to research in topics including plant competition, species richness and plant community ecology. His statistical research is equally diverse, covering such areas as permutation and bootstrap methods, path analysis, dynamic game theory and non-parametric regression smoothers. This rare combination of practical experience in both experimental science and statistical research makes him well positioned to communicate statistical methods to practising biologists in a meaningful way. Cause and Correlation in Biology A User’s Guide to Path Analysis, Structural Equations and Causal Inference BILL SHIPLEY Université de Sherbrooke, Sherbrooke (Qc) Canada           The Pitt Building, Trumpington Street, Cambridge, United Kingdom    The Edinburgh Building, Cambridge CB2 2RU, UK 40 West 20th Street, New York, NY 10011-4211, USA 477 Williamstown Road, Port Melbourne, VIC 3207, Australia Ruiz de Alarcón 13, 28014 Madrid, Spain Dock House, The Waterfront, Cape Town 8001, South Africa http://www.cambridge.org © Cambridge University Press 2004 First published in printed format 2000 ISBN 0-511-01772-3 eBook (netLibrary) ISBN 0-521-79153-7 hardback ISBN 0-521-52921-2 paperback À ma petite Rhinanthe, David et Élyse. Contents Preface xi 1 Preliminaries 1.1 The shadow’s cause 1.2 Fisher’s genius and the randomised experiment 1.3 The controlled experiment 1.4 Physical controls and observational controls 1 1 7 14 16 2 From 2.1