Appraising the evidence: what is selection bias?

Selection bias in epidemiological studies occurs when there is a systematic difference between the characteristics of those selected for the study and those ... Skiptomaincontent YouarehereHome Archive Volume10, Issue3 Appraisingtheevidence:whatisselectionbias? Emailalerts ArticleText Articlemenu ArticleText Articleinfo CitationTools Share RapidResponses Articlemetrics Alerts PDF EBMHnotebook Appraisingtheevidence:whatisselectionbias? MaxHenderson,LisaPageDepartmentofPsychologicalMedicine,InstituteofPsychiatry,London,UKCorrespondenceto:DrMHendersonDepartmentofPsychologicalMedicine,InstituteofPsychiatry,WestonEducationCentre,CutcombeRoad,LondonSE59RJ,UK;m.henderson{at}iop.kcl.ac.uk http://dx.doi.org/10.1136/ebmh.10.3.67 StatisticsfromAltmetric.com Almostallstudiesarepronetoerror—theyusesamplesdrawnfromapopulationtoestimatewhatisoccurringorwhatmightoccurinthewholepopulation.Theseerrorscanbroadlybedividedintotwo:randomerrorandsystematicerror.Randomerroristheplayofchanceandresultsinanestimateofeffect(forexample,relativerisk)beingequallylikelytobeaboveorbelowthetruevalue.Itsroleisassessedwithstatisticalmeasuressuchaspvaluesandconfidenceintervals.Systematicerroriscalledbias,andalsoleadstotheestimatebeingaboveorbelowthetruevalue.Systematicerrorcanbefurtherdividedintoinformationbias,whichrelatestothemisclassificationofdata,andselectionbias,whichisthefocusofthisarticle. BiasAnytrendinthecollection,analysis,interpretation,publicationorreviewofdatathatcanleadtoconclusionsthataresystematicallydifferentfromthetruth.(LastJ.Adictionaryofepidemiology,2001) Biaslimitstheconclusionsthatcanbedrawnfromananalysis.Itisparticularlyproblematicbecause,unlikeconfounding,littlecanbedoneto“allow”or“control”foritoncethedatahavebeencollected.Assuchitisinmanywaysanissueofstudydesign,planningandpractice.Selectionbiasinepidemiologicalstudiesoccurswhenthereisasystematicdifferencebetweenthecharacteristicsofthoseselectedforthestudyandthosewhoarenot.Italsooccursininterventionstudieswhentherearesystematic…ViewFullText RequestPermissions IfyouwishtoreuseanyorallofthisarticlepleaseusethelinkbelowwhichwilltakeyoutotheCopyrightClearanceCenter’sRightsLinkservice.Youwillbeabletogetaquickpriceandinstantpermissiontoreusethecontentinmanydifferentways. Copyrightinformation: Copyright2007bytheEvidence-BasedMentalHealth LinkedArticles EBMHnotebookIsEBMHtooPC? StevenReid Evidence-BasedMentalHealth2007;1066-66PublishedOnlineFirst:25Jul2007.doi:10.1136/ebmh.10.3.66AetiologyCannabisuseincreasesriskofdevelopingsymptomsofmania BMJPublishingGroupLtd,RoyalCollegeofPsychiatristsandBritishPsychologicalSociety Evidence-BasedMentalHealth2007;1061-61PublishedOnlineFirst:25Apr2007.doi:10.1136/ebmh.10.2.61 ReadthefulltextordownloadthePDF: Subscribe(£96) Login LoginviaInstitutionLoginviaOpenAthens Loginusingyourusernameandpassword ForpersonalaccountsORmanagersofinstitutionalaccounts Username* Password* Forgotyourlogindetails?Registeranewaccount?Forgotyourusernameorpassword? ReadthefulltextordownloadthePDF: Subscribe(£96) Login LoginviaInstitutionLoginviaOpenAthens Loginusingyourusernameandpassword ForpersonalaccountsORmanagersofinstitutionalaccounts Username* Password* Forgotyourlogindetails?Registeranewaccount?Forgotyourusernameorpassword?