Epidemiologia

Basic Statistics and Epidemiology
A practical guide

Antony Stewart
Senior Lecturer in Public Health School of Health University of Wolverhampton

Radcliffe Medical Press

Radcliffe Medical Press Ltd 18 Marcham Road Abingdon Oxon OX14 1AA United Kingdom www.radcliffe-oxford.com The Radcliffe Medical Press electronic catalogue and online ordering facility. Direct sales to anywhere in theworld.

© 2002 Antony Stewart All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior permission of the copyright owner. Every attempt has been made to ensure that the information provided in this book is correct at the time ofprinting. This does not diminish the requirement to exercise judgement, and neither the publishers nor the author can accept responsibility for its use in practice. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 1 85775 589 8

Typeset by Joshua Associates Ltd, Oxford Printed and bound by TJ International Ltd, Padstow,Cornwall

Contents
Foreword Preface Acknowledgements 1 What are statistics? 2 Populations and samples 3 Random sampling 4 Presenting data 5 Frequencies, percentages, proportions and rates 6 Types of data 7 Mean, median and mode 8 Gentiles 9 Standard deviation 10 Standard error 11 Normal distribution 12 Confidence intervals 13 Probability 14 Hypothesis tests and P-values 15 The Mest 16 Parametricand non-parametric tests v vi vii 1 3 7 11 15 19 23 27 29 33 35 39 43 47 51 57

iv • Contents

17 Correlation and regression 18 Chi-squared test 19 Statistical power and sample size 20 22 24 25 26 27 28 29 What is epidemiology? Measuring disease frequency Prevalence studies Questionnaires Cohort studies Case-control studies Randomised controlled trials Screening 21 Bias and confounding 23Measuring association in epidemiology

59 61 67 69 71 75 83 91 93 99 103 107 113 121 123 129 135 143 145 147

Glossary of terms Appendix 1 Statistical tables Appendix 2 Exercises Appendix 3 Answers to exercises References Further reading Index

Foreword
Understanding about risks to people's health and their relationships with aspects of life is increasingly an essential part of daily life. Newreports, new potential hazards and possibilities for prevention arise all the time. What do they mean, can they be reliable, and how can we know? We are all increasingly getting to grips with the essential elements of epidemiological methods and it is happening anyway to us all. What is an odds ratio and how do we interpret such a thing? How is it measured or estimated? What makes a good studyand what do we look out for to detect rubbish or bias in epidemiology? Is the third generation pill better or worse than its predecessors? Why do I need a mammography test? What good is it doing? How can we manipulate relative risks from one study to another and from one risk factor to another? Why incidence and not prevalence? And how do we measure variation? What assumptions might we be making? Isthis questionnaire going to give reliable answers for the purpose intended? All this and more in a readable and logical form is presented in this book. Clearly, as a basic text it is invaluable but for the lay reader who wants to know the elements of epidemiological method, it will be very useful too. The more earnest even have a selection of examples to work through, with answers given. In theend, a better life free of disease depends a lot on us all being able to avoid the avoidable determinants of illness - possibly 40 years before the risk is even appreciable. All that will depend on us knowing the consequences of not doing so. That is Basic Statistics and Epidemiology. Klim McPherson Professor of Public Health Epidemiology Medical Research Council Department of Social Medicine...