How to read a paper: BMJ


HOW TO READ A PAPER

The basics of evidence based medicine
Second edition
TRISHA GREENHALGH
Department of Primary Care and Population Sciences
Royal Free and University College Medical School
London, UK
© BMJ Books 2001

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First published in 1997
Second impression 1997
Third impression 1998
Fourth impression 1998
Fifth impression 1999
Sixth impression 2000
Seventh impression 2000
Second Edition 2001
by the BMJ Publishing Group, BMA House,Tavistock Square, London WC1H 9JR
http://www.bmjbooks.com
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 0-7279-1578-9
Cover by Landmark Design, Croydon, Surrey
Typeset by FiSH Books, London
Printed and bound by MPG Books Ltd, Bodmin

Contents
Foreword to the first edition ix
Preface xiii
Preface to the first edition: Do you need
to read this book? xv
Acknowledgments xvii
1 Why read papers at all? 1
Does “evidence based medicine” simply mean
“reading medical papers”? 1
Why do people often groan when you mention evidence
based medicine? 3
Before you start: formulate the problem 8
2 Searching the literature 15
Reading medical articles 15
The Medline database 16
Problem 1:You are trying to find a particular paper
which you know exists 17
Problem 2:You want to answer a very specific
clinical question 22
Problem 3:You want to get general information quickly
about a well defined topic 25
Problem 4:Your search gives you lots of irrelevant
articles 29
Problem 5:Your search gives you no articles
at all or not as many as you expected 30
v
Problem 6:You don’t know where to start searching 32
Problem 7:Your attempt to limit a set leads to loss of
important articles but does not exclude those of low
methodological quality 33
Problem 8: Medline hasn’t helped, despite a thorough
search 34
The Cochrane Library 36
3 Getting your bearings (what is this paper about?) 39
The science of “trashing” papers 39
Three preliminary questions to get your bearings 41
Randomised controlled trials 46
Cohort studies 50
Case-control studies 51
Cross-sectional surveys 52
Case reports 53
The traditional hierarchy of evidence 54
A note on ethical considerations 55
4 Assessing methodological quality 59
Was the study original? 59
Who is the study about? 60
Was the design of the study sensible? 62
Was systematic bias avoided or minimised? 64
Was assessment “blind”? 68
Were preliminary statistical questions addressed? 69
Summing up 73
5 Statistics for the non-statistician 76
How can non-statisticians evaluate statistical tests? 76
vi
Have the authors set the scene correctly? 78
Paired data, tails, and outliers 83
Correlation, regression and causation 85
Probability and confidence 87
The bottom line (quantifying the risk of benefit and harm) 90
Summary 92
6 Papers that report drugs trials 94
“Evidence” and marketing 94
Making decisions about therapy 96
Surrogate endpoints 97
How to get evidence out of a drug rep 101
7 Papers that report diagnostic or screening tests 105
Ten men in the dock 105
Validating diagnostic tests against a gold standard 106
Ten questions to ask about a paper which claims to
validate a diagnostic or screening test 111
A note on likelihood ratios 116
8 Papers that summarise other papers
(systematic reviews and meta-analyses) 120
When is a review systematic? 120
Evaluating systematic reviews 123
Metaanalysis for the non-statistician 128
Explaining heterogeneity 133
9 Papers that tell you what to do (guidelines) 139
The great guidelines debate 139
Do guidelines change clinicians’ behaviour? 141
Questions to ask about a set of guidelines 144
vii
10 Papers that tell you what things cost
(economic analyses) 151
What is economic analysis? 151
Measuring the costs and benefits of health interventions 153
Ten questions to ask about an economic analysis 158
Conclusion 163
11 Papers that go beyond numbers
(qualitative research) 166
What is qualitative research? 166
Evaluating papers that describe qualitative research 170
Conclusion 176
12 Implementing evidence based findings 179
Surfactants versus steroids: a case study in adopting evidence
based practice 179
Changing health professionals’ behaviour: evidence
from studies on individuals 181
Managing change for effective clinical practice: evidence
from studies on organisational change 188
The evidence based organisation: a question of culture 189
Theories of change 193
Priorities for further research on the implementation
process 195
Appendix 1: Checklists for finding, appraising, and
implementing evidence 200
Appendix 2: Evidence based quality filters for
everyday use 210
Appendix 3: Maximally sensitive search strings
(to be used mainly for research) 212
Appendix 4: Assessing the effects of an intervention 215
Index 216

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