determining sample size - eans · pdf filefeasibility studies ... summary – determining...

10-07-2016

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Determining Sample Size considera3ons for interven3on studies & surveys

Dr.BetsievanGaal

Introduc3on Sample Sizes

• Whyisitimportanttoconsidertherequiredsamplesizeforastudy?

EuropeanAcademyofNursingScience–2016 2

10-07-2016

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Introduc3on Sample Sizes

• Whyisitimportanttoconsidertherequiredsamplesizeforastudy?

EuropeanAcademyofNursingScience 2hHp://freshspectrum.com

Objec3ves of this lecture

• UnderstandingoftheprinciplesofsamplesizesintermsofevaluaLngcomplexintervenLons

• UnderstandingofdifferentmethodsofcalculaLngsamplesizesappropriateforevaluaLngcomplexintervenLons

• UnderstandingthecontribuLonofclustersincalculaLngsamplesizes

EuropeanAcademyofNursingScience 3

10-07-2016

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What are important concepts to know

• ResearchpopulaLon• Asample•  Samplescheme• Design


Researchproblem

Researchques0on Methodology

Importance of good sample sizes

• RepresentaLveness:doesyoursamplerepresentthepopulaLon?

•  SelecLonbias:over/underrepresenLngofsomesubgroups

• GeneralisaLon:canyougeneralisetheresultsofthesampletothepopulaLon?


PopulaLons&SamplesThefocusisonapopulaLonthoughwecanonlyexaminesamples

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Sample size methods

1.  Probabilitysampling–aselect/arandomsample

2.  Non-probabilitysampling–select/anonrandomsample??


Sample size: 1. probability sampling

When?•  ListofthewholepopulaLon• GeneralisaLonisimportant

• OYenquanLtaLvestudies


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Sample size: 1. probability sampling

1.  Asimplerandomsampling2.  StraLfiedrandomsampling3.  Clusterrandomsampling4.  SystemaLcrandomsampling


Sample size: 1. probability sampling 1 a simple random sampling

• AllpeopleinthepopulaLonhavethesamechangetobeselected•  Forexample:

•  PaLents•  Students


Polit,D.F.&Beck,C.T.(2012,p.208).Nursingresearch.Genera/ngandassessingevidencefornursingprac/ce(ninthediLon).Philadelphia:LippincoHWilliams&

Wilkins.

Small table of random digits

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Sample size: 1. probability sampling 2 Stra3fied random sampling

• PopulaLonsdividedintosubpopulaLons:strata•  Forexample:socialeconomicstatus/school


Burns,A.C.&Bush,R.F.(2011).Principesvanmarktonderzoek.ToepassingenmetSPSS.Amsterdam:PearsonBeneluxB.V.

Sample size: 1. probability sampling 3 cluster random sampling

• PopulaLonsdevidedintosubpopulaLons:strata•  Forexample:socialeconomicstatus/school


CenterforstaLsLcsUniversityofHasselt,Belgium

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Sample size: 1. probability sampling 4 Systema3c random sampling

• WholepopulaLons


CenterforstaLsLcsUniversityofHasselt,Belgium

Summary: 1. probability sampling

• GeneralizetothepopulaLon•  LesschangeofselecLonbias,butnoguaranty•  LesspracLcal–Costly&Lmeconsuming


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Sample size: 2. non probability sampling

When?• GeneralisaLontothewholepopulaLonisnotimportant• Notenoughresources:money,Lme•  TracingthepopulaLonisdifficult???

• UsedforquanLtaLve&qualitaLvestudies


Sample size: 2. non probability sampling

1.  Conveniencesampling2.  Snowballsampling3.  Quotasampling4.  Purposefulsampling


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Sample size: 2. non probability sampling 1 Convenience sampling

•  Alsocalledaccidentalsampling•  Surveyamongstudentsofanuniversityyouworkwith•  Surveyamongnursingofahospitalyouknow•  Surveyamongpeoplewhoareinoneshoppingarea


Sample size: 2. non probability sampling 4 Snowball sampling

•  Alsocalledchainsampling


hHp://www.wordpress.com

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Sample size: 2. non probability sampling 3 Quota sampling

•  Aconveniencesamplingwithinastrata•  StrataisdefinedbydataofthepopulaLon

•  Gender•  EducaLonlevel•  Medicaldisease


Strata

Conveniencesampling

Sample size: 2. non probability sampling 3 Quota sampling


Strata Popula0on Conveniencesampling Quotasampling

Men 100(20%) 5(5%) 20(20%)

Women 400(80%) 95(95%) 80(80%)

Total 500(100%) 100(100%) 100(100%)

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Sample size: 2. non probability sampling 2 Purposeful sampling

•  When:•  Selectspecificpeopleinyourstudy•  ForexampletesLnganewintervenLon

EuropeanAcademyofNursingScience 4hHp://www.memrise.com

Summary: 2. non probability sampling

• ArepracLcal• BeawareofselecLonbias• ComparethesamplewiththepopulaLon


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Calcula3ng sample sizes

Why?• PracLcalconsideraLons:limitedbudget/subjects•  Toensureameaningfulresultofthestudy• Avoidsstudiesthatare

•  toosmall(underpowered)•  solargethattheydetecteffectsthatarenotrelevant(overpowered)



Because:•  ThefocusisoYenonapopulaLonthoughtypicallywecanonlyexaminesamples

•  ThetaskofstaLsLcsisto:•  EsLmatetheproperLesofapopulaLonfromthoseofthesample•  EsLmatetheeffectofanintervenLoninapopulaLonfromtheeffectinasample


TalkwithyourstaLsLcianaboutcalculaLngthesamplesize

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Factorsthatinfluencethepowerofastudy• Desiredpowerlevel–ErrorsintesLnghypothesis:TypeI/II•  EsLmatedeffectsize• Designofthestudy• Measurementlevel•  Theoutcome• Analysistechniques




Decisionbasedonsta0s0caltestUnknownifhypothesisistrue Notreatmenteffect

Treatmenteffectexists

Notreatmenteffect Correctdecisionp=1–α

TypeIerrorp=α

Treatmenteffectexists

TypeIIerrorp=β

Correctdecisionp=1–β=Power

TheoutcomeofastaLsLcaltestandtheirprobabiliLes

PowerCorrectlyrejectafalsehypothesis

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Example: Washing Without Water study


Aims •  To compare skin damage •  To compare costs

Interven

0on

Tradi0onalBathing

WashingwithdisposableWashinggloves

Control

Work in groups

ReadthesummaryoftheWashingWithoutWaterstudyandanswerthenextquesLonswithyourgroup.1.  Whatsamplesizemethodshasbeenusedinthatstudy?2.  WhatinformaLondoyouneedtocalculatethesamplesizebefore

thestartofthisstudy?


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Work in groups

1.  Samplesizemethod:

2.  NeededinformaLontocalculatethesamplesize:


Sample size & the MRC framework


DevelopmentIdenLfyingtheevidencebaseIdenLfyingordevelopingtheoryModellingprocessandoutcomes

Feasibilityandpilo0ngTesLngproceduresEsLmaLngrecruitmentandretenLonDeterminingsamplesize

Evalua0onAssessingeffecLvenessUnderstandingchangeprocessAssessingcosteffecLveness

Implementa0onDisseminaLonSurveillanceandmonitoringLongtermfollow-up(CraigPetal.2008)

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Sample size & the MRC frame work Development

•  IdenLfytheimportanteffect–theoutcome–andhowtomeasures• MakingplausibleesLmateoflikelyeffect•  IdenLfyingexisLngstudiesfromwhichesLmatesoflikelyeffectandsamplingvariabilitycanbedrawn

•  EsLmaLngtheprevalenceofaproblem•  Findingstudysubjects•  Impact/returnoninvestment


Sample size & the MRC frame work Feasibility studies

•  InsufficientpriorinformaLontocalculatesamplesizerequired…• Gatheringdataaboutvariabilityininstrumentsetc.fordefiniLveevaluaLon:

•  Assessingfeasibilityofrecruitmentanddropoutsthatwilleffectsamplesize•  EsLmatesofvariability(SD)ofthepopulaLon•  IdenLfyingbaselinerates

•  TesLngstudyprocedures


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Sample size & the MRC frame work Defini3ve evalua3on

• PowercalculaLonrequiredinadvancefordefiniLvestudy• Mustdefinemainendpointandtheprecisionrequired(smallesteffectthatisimportant):

•  ‘Posthoc’powercalculaLons(whatthepowerwas,aYerthestudy)aremeaningless


Some specific considera3ons

• Cluster(randomised)studies•  StandardtechniquesdonottakeintoaccountcorrelaLonsbetweenindividualswithinclusters

•  Largersamplesarerequired

• CosteffecLveness•  VariaLonoccurson2parameters:costandeffect•  EffectonsamplesizevariesdependingonthestrengthanddirecLonofthecorrelaLon


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Summary – determining sample size

•  Inthislectureshowedthe• DifferentmethodstosampleyourstudypopulaLonfromthepopulaLons

•  TheimportanceofapropersamplesizecalculaLon•  TheimportancetomeetyourstaLsLciananddecidewhatthebestsamplesizeisforyourstudyas

•  Onwsizedoesnoetfitall,aseelding


LectureSamplesize: WorkinginGroups 12–07–2016

WashingWithoutWater–study

Readthesummaryofthisstudyandanswerthenextquestions:

1. Whatsamplesizemethodshadbeenusedinthisstudy?

2. Whatinformationdoyouneedtocalculatethesamplesizebeforethestartofthisstudy?

Background:No-rinsedisposablewashglovesareincreasinglyimplementedinhealthcaretoreplace

traditionalsoapandwaterbedbathswithoutproperevaluationof(cost)effectiveness.

Objectives:Tocomparebedbathsforeffectsonskinintegrityandresistanceagainstbathingandcosts.

Design:Clusterrandomizedtrial.Randomizationwasperformedpriortobaselinedatacollectionandat

thelevelofnursinghomewards.Residentswithinthesamewardwereconsideredtobeacluster

Setting&Participants:Fivehundredadultcare-dependentresidentsfromnursinghomewards.

Methods:Theexperimentalcondition‘washingwithoutwater’consistsofabedbathwithdisposable

washglovesmadeofnon-wovenwaffledfibers,saturatedwithano-rinse,quicklyvaporizingskincleaning

andcaringlotion.Thecontrolconditionisatraditionalbedbathusingsoap,water,washclothsand

towels.Bothconditionswerecontinuedfor6weeks.

Theprimaryoutcomeforthisstudywasprevalenceofskindamage,definedasclinicalsymptomsof

intertrigo,(contact)dermatitis,orcandidiasis.Wedistinguishedtwolevelsofseverityofskindamage:

anyskinabnormality/lesionandsignificantskinlesions:1)Anyskinabnormalities/lesionswereconsidered

presentwhenatleastoneofthefollowingsymptomswereobserved:brightreddiscoloration;erythema;

white,greenoryellowdiscolorationofthewoundbed;atrophicandshinyskin;satellitelesionsandkissing

lesions,fissures,erosionsorulcerations.Thesesymptomsshouldbepresentonthebuttocksoroneofthe

following10skinfolds:eyes,neck,armpits,elbows,sub-mammaryregion,umbilicus,abdomen,groins,

analcleft,andbetweentoes.2)Significantskinlesionswereconsideredpresentiftheskinwasnotintact

onthebuttocksoroneofthefollowingfourskinfolds:sub-mammaryregion,abdomen,groins,andanal

cleft.

Additionaloutcomes:resistanceduringbedbaths,costs.

Results:Anyskinabnormalities/lesionsovertimedecreasedslightlyintheexperimentalgroup,and

increasedslightlyinthecontrolgroup,resultingin72.7%vs77.6%ofresidentshavinganyskin

abnormalities/lesionsafter6weeks,respectively(p=0.04).Therewerenodifferencesinsignificantskin

lesionsorresistanceafter6weeks.

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