Medical Statistics A Guide to Data Analysis and Critical Appraisal Jennifer Peat Associate ...

338 Pages·2007·2.87 MB·English

Medical
Statistics
A Guide to Data Analysis
and Critical Appraisal
Jennifer Peat
Associate Professor, Department of Paediatrics and Child Health, University
of Sydney and Senior Hospital Statistician, Clinical Epidemiology Unit, The
Children’sHospitalatWestmead,Sydney,Australia
Belinda Barton
Head of Children’s Hospital Education Research Institute (CHERI) and
Psychologist, Neurogenetics Research Unit, The Children’s Hospital at
Westmead,Sydney,Australia
ForewordbyMartinBland,ProfessorofHealthStatisticsattheUniversityof
York

Medical Statistics
A Guide to Data Analysis and Critical Appraisal

Medical
Statistics
A Guide to Data Analysis
and Critical Appraisal
Jennifer Peat
Associate Professor, Department of Paediatrics and Child Health, University
of Sydney and Senior Hospital Statistician, Clinical Epidemiology Unit, The
Children’sHospitalatWestmead,Sydney,Australia
Belinda Barton
Head of Children’s Hospital Education Research Institute (CHERI) and
Psychologist, Neurogenetics Research Unit, The Children’s Hospital at
Westmead,Sydney,Australia
ForewordbyMartinBland,ProfessorofHealthStatisticsattheUniversityof
York
(cid:2)C 2005byBlackwellPublishingLtd
BMJBooksisanimprintoftheBMJPublishingGroupLimited,usedunderlicence
BlackwellPublishingInc.,350MainStreet,Malden,Massachusetts02148-5020,USA
BlackwellPublishingLtd,9600GarsingtonRoad,OxfordOX42DQ,UK
BlackwellPublishingAsiaPtyLtd,550SwanstonStreet,Carlton,Victoria3053,Australia
TherightoftheAuthortobeidentifiedastheAuthorofthisWorkhasbeenassertedin
accordancewiththeCopyright,DesignsandPatentsAct1988.
Allrightsreserved.Nopartofthispublicationmaybereproduced,storedinaretrieval
system,ortransmitted,inanyformorbyanymeans,electronic,mechanical,photo-
copying,recordingorotherwise,exceptaspermittedbytheUKCopyright,Designs
andPatentsAct1988,withoutthepriorpermissionofthepublisher.
Firstedition2005
LibraryofCongressCataloging-in-PublicationData
Peat,JenniferK.
Medicalstatistics:aguidetodataanalysisandcriticalappraisal/byJennifer
PeatandBelindaBarton.–1sted.
p.;cm.
Includesbibliographicalreferencesandindex.
ISBN-13:978-0-7279-1812-3
ISBN-10:0-7279-1812-5
1.Medicalstatistics. 2.Medicine–Research–Statisticalmethods.
I.Barton,Belinda.II.Title.
[DNLM:1.Statistics–methods. 2.ResearchDesign. WA950P363m2005]
R853.S7P432005
610(cid:3).72(cid:3)7–dc22 2005000168
AcataloguerecordforthistitleisavailablefromtheBritishLibrary
Setin9.5/12ptMeridien&FrutigerbyTechBooks,NewDelhi,India
PrintedandboundinHarayana,IndiabyReplikaPressPvtLtd
CommissioningEditor:MaryBanks
EditorialAssistant:MirjanaMisina
DevelopmentEditor:VeronicaPock
ProductionController:DebbieWyer
ForfurtherinformationonBlackwellPublishing,visitourwebsite:
http://www.blackwellpublishing.com
Thepublisher’spolicyistousepermanentpaperfrommillsthatoperateasustainable
forestrypolicy,andwhichhasbeenmanufacturedfrompulpprocessedusingacid-free
andelementarychlorine-freepractices.Furthermore,thepublisherensuresthatthetext
paperandcoverboardusedhavemetacceptableenvironmentalaccreditationstandards.
Contents
Foreword,vii
Acknowledgements,ix
Chapter1 Datamanagement:preparingtoanalysethedata,1
Chapter2 Continuousvariables:descriptivestatistics,24
Chapter3 Continuousvariables:comparingtwoindependentsamples,51
Chapter4 Continuousvariables:pairedandone-samplet-tests,86
Chapter5 Continuousvariables:analysisofvariance,108
Chapter6 Continuousdataanalyses:correlationandregression,156
Chapter7 Categoricalvariables:ratesandproportions,202
Chapter8 Categoricalvariables:riskstatistics,241
Chapter9 Categoricalandcontinuousvariables:testsofagreement,267
Chapter10 Categoricalandcontinuousvariables:diagnosticstatistics,278
Chapter11 Categoricalandcontinuousvariables:survivalanalyses,296
Glossary,307
Index,317
v

Foreword
Most research in health care is not done by professional researchers, but by
health-care practitioners. This is very unusual; agricultural research is not
done by farmers, and building research is not done by bricklayers. I am told
that it is positively frowned upon for social workers to carry out research,
whentheycouldbesolvingtheproblemsoftheirclients.Practitioner-ledre-
searchcomesabout,inpart,becauseonlyclinicians,ofwhateverprofessional
background, have access to the essential research material, patients. But it
also derives from a long tradition, in medicine for example, that it is part of
theroleofthedoctortoaddtomedicalknowledge.Itisimpossibletosucceed
in many branches of medicine without a few publications in medical jour-
nals.Thistraditionisnotconfinedtomedicine.LetusnotforgetthatFlorence
Nightingalewasknownas‘thePassionateStatistician’andhergreatestinno-
vationwasthatshecollecteddatatoevaluatehernursingpractice.(Shewas
the first woman to become a fellow of the Royal Statistical Society and is a
heroinetoallthinkingmedicalstatisticians.)
Thereareadvantagestothissystem,especiallyforevidence-basedpractice.
Clinicians often have direct experience of research as participants and are
aware of some of its potential and limitations. They can claim ownership of
theevidencetheyareexpectedtoapply.Thedisadvantageisthathealth-care
researchisoftendonebypeoplewhohavelittletraininginhowtodoitand
who have to do their research while, at the same time, carrying on a busy
clinical practice. Even worse, research is often a rite of passage: the young
researchercarriesoutoneortwoprojectsandthenmovesonanddoesnotdo
researchagain.Thusthereisacontinualstreamofnewresearchers,needing
tolearnquicklyhowtodoit,yetthereisashortageofseniorresearchersto
actasmentors.Andresearchisnoteasy.Whenwedoapieceofresearch,we
are doing something no one has done before. The potential for the explorer
tomakeajourneywhichleadsnowhereisgreat.
The result of practitioner-led research is that much of it is of poor quality,
potentiallyleadingtofalseconclusionsandsub-optimaladviceandtreatment
for patients. People can die. It is also extremely wasteful of the resources
of institutions which employ the researchers and their patients. From the
researchers’pointofview,readingthepublishedliteratureisdifficultbecause
thefindingsofotherscannotbetakenatfacevalueandeachpapermustbe
read critically and in detail. Their own papers are often rejected and even
oncepublishedtheyareopentocriticismbecausethemostcarefulrefereeing
procedureswillnotcorrectalltheerrors.
Whenresearchersbegintoreadtheresearchliteratureintheirchosenfield,
one of the first things they will discover is that knowledge of statistics is
vii
viii Foreword
essential. There is no skill more ubiquitous in health-care research. Several
ofmyformermedicalstudentshavecometomeforabitofstatisticaladvice,
tellingmehowtheynowwishedtheyhadlistenedmorewhenItaughtthem.
Well, I wish they had, too, but it would not have been enough. Statistical
knowledgeisveryhardtogain;indeed,itisoneofthehardestsubjectsthere
is, but it is also very hard to retain. Why is it that I can remember the lyrics
(though not, my family assures me, the tunes) of hundreds of pop songs of
myyouth,butnotthedetailsofanystatisticalmethodIhavenotappliedin
thelastmonth?AndIspendmuchofmytimeanalysingdata.
What the researchers need is a statistician at their elbow, ready to answer
any questions that arise as they design their studies and analyse their data.
Theyaresohardtofind.Evenoneconsultationwithastatistician,ifitcanbe
obtainedatall,mayinvolveawaitforweeks.Ithinkthatthemostefficientway
toimprovehealth-careresearchwouldbetotrainandemploy,preferablyat
highsalaries,largenumbersofstatisticianstoactascollaborators.(Incidentally,
statisticians should make the ideal collaborators, because they will not care
about the research question, only about how to answer it, so there is no
risk of them stealing the researcher’s thunder.) Until that happy day dawns,
statisticalsupportwillremainashardtofindasanhonestpolitician.Thisbook
providesthenextbestthing.
The authors have great experience of research collaboration and support
forresearchers.JennyPeatisastatisticianwhohasco-authoredmorethana
hundredhealthresearchpapers.Shedescribesherselfasa‘researchtherapist’,
always ready to treat the ailing project and restore it to publishable health.
Belinda Barton brings the researcher’s perspective, coming into health re-
searchfromabackgroundinpsychology.Theirpracticalexperiencefillsthese
pages. The authors guide the reader through all the methods of statistical
analysis commonly found in the health-care literature. They emphasise the
practicaldetailsofcalculation,givingdetailedguidanceastothecomputation
of the methods they describe using the popular program SPSS. They rightly
stresstheimportanceoftheassumptionsofmethods,includingthosewhich
statisticiansoftenforgettomention,suchastheindependenceofobservations.
Researcherswhofollowtheiradviceshouldnotbetoldbystatisticalreferees
thattheiranalysesareinvalid.PeatandBartoncloseeachchapterwithalistof
thingstowatchoutforwhenreadingpaperswhichreportanalysisusingthe
methodstheyhavejustdescribed.Researcherswillalsofindtheseinvaluable
ascheckliststousewhenreadingovertheirownwork.
I recently remarked that my aim for my future career is to improve the
quality of health-care research. ‘What, worldwide?’, I was asked. Of course,
whylimitourselves?Ithinkthatthisbook,comingfromtheothersideofthe
worldfromme,willhelpbringthattargetsomuchcloser.
MartinBland,
ProfessorofHealthStatistics,UniversityofYork,
August2004