study design part1_20jan2014

8/12/2019 Study Design Part1_20Jan2014

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@Case-controlstudies

- - --<-\)ntrol studycompares hecharacteristics fa group ofpatients

'-

i rirnicular disease utcome(thecases) o a group of individuals

. -_ -rr disease utcome(the controls)' to seewhether exposure o

-'_:iroroccurredmoreorlessirequently

nthecasesthanthecontrols: : :h.I ). Suchrerospective studiesdo not provide information on

:t :::r alenceor incidenceof disease ut may give cluesas o which

-.' :. elevate rreduce he iskof disease.

S€lection of cases-'< :lisibility criteria for casesshould be precise and unambiSuous

: : liebetes mellitus lWorld Hea]th Organjzationcriteriall single

. __ glucose concenlrr l i ,rn>7 mmol/l iLre Ur renous plasma

I --,\e measured2 hours after ingestion of 75g oral glucose load

:rmol/litre). [n particular,t is mportantto delinewhether ncident

.,<. pntientswho arerecruitedat the time ofdiagnosis) or prevalent

-,<. pirtientswho were alreadydiagnosed eforeentering he study)

.

-J be recruited.Prevalentcasesmay have had time to reflecl on

-.- : h'lory

of exposure o known risk actors.especally if thedi sease

, ,re l-publicized one such as cancer,and may have altered their

-r-:\ iour after diagnosis. t is importantto identify as many citsesas

:, -.rble so that lhe resultscany more weight and the conclusions an

'< .neralized to futurepopulations.To this end, t may be necessaryo

-::. 'hospital istsand diseaseegistr ies. nd to include aseswh o

- a.: Juring the time period when casesand controlswere recrulted'

_<::usetheir excluskrnmay lead o a biasedsarnple fcases.

Selection of controls: . * rrh uses.heeliEibil ir) r iterir for (onlrol. shnulJ lso he preci'e

-_J unambiguous.Controlsshould be screened t entry to the study o

:-.Lrre that they do not have the diseaseof interest.Where possible'

':trols shouldbe selectedrom the samesourceas casesControlsare

::an selected rom hospitals.However, as risk f-actorselated o one

::.eir\e outcome may also be related o other diseaseoutcomes' he

< .ction of hospital-based ontrols may over-select ndividuals whtr

_:\('been exposed o the risk lactorofinterest' andmay. herefore'not

-.i\ ir] s be appropriate. t is often acceptableo selectcontrols iom the

::neral population,although hey may not be as motivated o takepart

- .rr.ha stuJ).and e.pon\e r te\ mr) therelL'reepoorer n control\

':rn cases.The use of neighbourhood controls may ensure that

-i.cs and controls are fiom similar social backgrounds.Ol note, t is

:rportant b avoid the temptation to relax the criteria fbr eligibility

: ., 'nlrol\ pafl-ua) lhrough slud) impl) lt ' .peed up heprocets [

-aanrilment.

\lthough most case{ontrol studies ncludeonly a singlecontrol tbr:J.h casek)tien referred o asa I ll case-controlstudy). t is possible o

:i lude multiple controls or eachcase a l:n case{ontrol study)'

lncreasednumbers of controls per casewill provide the study with

:r. alerpower (Chapter 8), althoughany suchgains n powerare ikely

:,r be fairly srnall beyond four controls per casel Where a greater

:umber of individuals are eligible to be selectedas controls than is

s

Gdmes,D.A. andSchulz.K.F.(2005)Compared o what? Findingcontrols br

-r'e {ontrol studies. d,r.et, 365. 1429-31.

Pastime -- - - - - -< Presentime

Trace t

startinlgoint

Figure16.1 Diagrammaticcprcsentationfa case{ontrol stLldy.

rcquired. t is mportantk) documcnthow thecontrolsshouldbeselected

(c.g.bt random electioniom all el igible ndividuals)

ldent i f icat ion of r isk tactorsAs in irny epiclemiologicalstudy. he poteilial risk factors should be

delined betbreconducting hc study The delinition of lhese actors of

interestshould be clear and unambiguous e g in a case-controlstudy

for t he developmentofdiabetesmellitus,where exercise'is the factor

of interest, here shouldbe a clexr explanationot'how exercise s to bemeasuredand categorized).A pilot study may help to ensure hat the

definition will be feasible given the need to rely on retrospectively

collecreddataand/or nemory.Othertactorswhich may havean mpact

on theoutcome i.e-case{ontrol status), itherasconfounders Chapter

34) and/oreffect modifiers,should alsobe listed anddefined.

MatchingMany case-control studiesare matched in order to selectcasesand

controls who are as similar as possible. we may have frequency

matching on ag/orp basis i.e. he average alueofeachofthe relevant

potentidl risk lactors of the whole group ol casesshouldbe similar lo

that ofthe whole group ofcontrols) orwe may havepairwise matching

on an n.1ivi../Ila/basisi.e.eachcases matched ndividually to a control

who has similar potential risk factors.).n general.when perlbrming

individualmatching,t s useful o sexmatch ndividualsi .e il thecase

is male, he control should alsobe maie),and.sometimes. atientswill

be agc matched.However, t is important not b matchon lhe basisof

the isk factorofinterest.oron any actor hat alls on the causalpathway

of the disease Chapter34), as his will remove he ability of the study

to assessany relationship between the risk factor and the disease'

Furthernore, it is important not to match on t()o manv tactors' as his

may rest ct the availabil i ty of suitablecontrols Untbrtunately'

matchingdoesmean hat heeflect on diseasc fthe variables hathave

beenused br matchingcannotbe studied

Case-controlstudies Study design 47

study design part1_20jan2014

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