ASSESSMENT OF CONFOUNDERS AND INTERACTIONS: AN ANALYSIS OF EPIDEMIOLOGICAL DATA.

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HelenKeller INTERNATIONAL

The

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Acollaborative program with the HarvardSchoolofPublic Health (Harvard University, Boston, USA), SEAMEO

RegionalCentreforFood and Nutrition (University of Indonesia,Jakarta, Indonesia), University ofMataram

(Mataram, Indonesia), University of Andalas (Padang, Indonesia), Helen Keller International-Indonesia (Jakarta,

Indonesia) and the Summit Institute of Development (Mataram, Indonesia) withsupportfrom

the United States Agency for International Development-Indonesia

;

data-driven

lesson

plans

IN

NUTRITIONand

PUBLIC

HEALTH

A

HANDBOOK

Editors:

Judhiastuty

Februhartanty

Airin

Roshita

Heida Khusun

Anuraj

Shankar

summit

SD1

US

AID

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AUTHORS

OF

THE LESSON PLANS

(inalphabetical order)

AirinRoshita

SEAMEORECFONUniversitas Indonesia

11. Judhiastuty Februhartanty

SEAMEO RECFON UniversitasIndonesia

2. AnitaShankar

JohnsHopkins University, Bloomberg

SchoolofPublic Health,USA 3. Aria Kekalih

Facultyof Medicine,Universitas Indonesia

12. LianaSuryaningsih

FacultyofAgriculture,Universityof

Mataram,NusaTenggara Barat,Indonesia

13. Lina Rospita

4. DefrimanDjafri

Facultyof Public Health, Andalas

University, Padang,Indonesia

5. DeniElnovriza

Facultyof Public Health,Universityof Andalas,Padang,Indonesia

6. Dian NBasuki

World FoodProgramIndonesia

Dwi Gayatri

FacultyofPublic Health,Universitas Indonesia

Helda Khusun

14. Luh AdeAriWiradnyani

SEAMEO RECFONUniversitasIndonesia

15. Nur Indrawaty Lipoeto

16. Rosvita Rasyid

FacultyofPublic Health, Andalas

17, SuryaHadi

FacultyofMathematics and Natural

Sciences, UniversityofMataram,Nusa

Tenggara Barat,Indonesia

18. Umi Fahmida

SEAMEO RECFONUniversitasIndonesia

9. Helmizar

10. IdralPurnakarya

19. ViviTriana

FacultyofPublic Health, Andalas

University,Padang,Indonesia

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USAID

summit

KfiU»r

HENRIData-driven lesson plans ) 17 Author;

DefrirnanDjafri (Faculty of Public Health,Andalas University,Padang, Indonesia)

Background

Confounding isa very important concept inepidemiology, because of itseffecton distortingthe association

betweenexposure andoutcome.Theconfoundingeffectoreffect modification (interaction)mustbeconsidered

.vhen anextraneousvariableaffects theassociation between the exposureand theoutcomeof interest. The

confoundingeffectcanbecontrolledby planningthestudywellpriortodata collection,orif the datais already

collectedwe cancontrolfor theconfoundingeffectby performingadjustedanalysis usingstratifiedor regression analysis.

Learningobjectives

Atthe end of thelesson,theparticipantsareableto:

1. understandtheconceptofconfoundingand

interaction/effect

modification

2. describetheprincipieandmethods employed in controllingforconfoundingfactors

3. identify a potentialconfounder

4. assesstheoccurrenceof confounders and effect modifiersby meansofstatistical analysis

Jsagewithin thecurriculum

~-iislesson ispartofthecourse "AnalyticalEpidemiology" for theUndergraduateProgramintheFacultyofPublic

-ealth, Andalas University,given insemesterVI. The data in this lessonisusedbyfacilitatorsto supportthe

ecturesand tasks. Thislesson may also beofferedin a postgraduate programwhereparticipants maybegiven

more independent tasks such as performing analysis and interpretation using raw data from actual

;_{jrveys/studies with}somestatisticalapplicationsuchas R, Epi Info, SPSS, STATA,etc,aswellas presentingthe -esuitsina plenary.

E gibilityofparticipants

~->e

participantsareundergraduatestudentsat FacultyofPublic HealthmajoringinEpidemiology. Participants mould havea basic knowledge in biostatisticsandepidemiology as well as a basicskills inusing a statistical

jftware.

4.

ASSESSMENT

OF

CONFOUNDERS AND

INTERACTIONS:

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Materials and resources :: ; _rT

Presentationslideson"Confounding"and"Effect

Modifier/interaction"

Anarticle aboutconfounding

DatasetforlectureonConfounding: LungCancer.xls

DatasetforTask2:CMD.xis

Rsoftware/Epi Info7 (opensourcesoftware) Worksheetfor Task 1(Appendix4.1)

Worksheet forTask 2 (Appendix 4.2)

Facilitators

The facilitatorsarethecourse coordinator(s).

Duration

Thislessonistobeconducted foratotal of2 sessions,each100 minutes.

Directionsofclass activities

1. In Session 1, facilitators complete the slide presentation on "Confounding". The dataset on

Lunj

Cancer.xlsisusedaspartofthe lecture.

2. Afterthelecture,participants are requiredtostudy anarticleaboutconfounding by Sonis (1998).

3. Participants are givenindividual takehomeassignmentsonTask1(Appendix 4.1)tocomputeandasses confounding in a given case study.

4. InSession2,facilitatorscompletethesiidepresentationon"Effect

Modifier/Interaction".

5. Participantsare givenindividual take homeassignmentsonTask2 (appendix 4.2) basedonthedataset

CHD.xisusingRsoftwareor EpiInfo7. Evaluationoftheparticipants

The evaluation of the participants in this lesson will contribute tothe overall final grade. The

following

componentsareused:

•

Individual tasks : 20%

•

Midtermexamination: 30%

•

Final examination : 50%

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i

mssm

valuation ofthe lesson

raluationisdoneonceatthe endof theSemester.Studentsare requestedtofill ina surveyquestionnaireto i/aluatethe overallcourse.The results ofthe survey

will

be shared with thecoursecoordinators andteaching

>amforimprovement.

efereoces

1. RosenbergDandHandlerA. Analytic epidemiologyand multivariable methods.NewYork:Springer-Verlak

inc., 1998.

2. RothmanKJ,GreenlandS,LashTL.Modernepidemiology,

3rd.

LippincottWilliams&Wilkins,2008

3. Savitz DA. Interpretingepidemiologic evidence: Strategiesforstudy designandanalysis.OxfordUniversity Press, 2003.

4. SonisJ.Acloser lookatconfounding. Fam Med 1998;30:584-588.

5. TwiskJWR. Appliedlongitudinal data analysisforepidemiology.Cambridge University Press, 2003.

6. Vittinghoff E,GliddenD,McCullochCE,ShiboskiS.Multi-predictormethods inbiostatistics.Universityof California,2002.

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Appendix4.1. WorksheetTask1onconfounding

TASK 1

Name

IDno:

Acohortstudywasconductedtoassessthe relationship betweenair contaminationexposures(high exposure anc

low exposure) and the occurrence of bronchitis. Itwas believed that smoking status may be a confounder. Therefore, thisstudyneedstofindoutifsmokingstatusis aconfounderintheairpollution-bronchitisassociation.

The totalsubjects inthestudy were 2648andconsistedof peoplewho experienced anddidnotexperience

ani

occurrenceofbronchitis. Around1307peoplewereexposedtohighaircontamination.Thestudyshowsthat 257 subjectshad bronchitis. Theproportionofsubjectsexposedtohigh air contamination butnothavingbronchitis

was

1129/2391.

Questions

subjects

who hadbronchitis andwerenotexposedto

high

air contamination?

Drawa2x2 tableto describe the relationship betweenair contaminationexposureandbronchitis

What is the proportion of subjects who were exposed to high air contamination and did not

ha\J

bronchitis?

3. Whatistheproportionofsubjectswho had bronchitis andwerenot exposed tohighaircontamination?

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4. Among subjects who did not have bronchitis, what is the proportion of them exposed to high air

contamination?

5. Amongstthesubjectswhowere exposedtohigh air contamination, what istheratioofthosewho hador

didnothave bronchitis?

6. What is ratioof respondents who were exposed ornotexposedtohigh aircontaminationwhosuffered frombronchitis?

7. Whatisthecorrectmeasurefor

association/risk

inthis study?Basedonthe relationship between

levels

of

air contamination exposureand bronchitis,calculatethe estimateofthis association andinterpretthe

results.

Fromthe results above, stratification by smoking status was madeto see whether or not itdistorts the relationship betweenexposure of air contamination and bronchitis, it was found that the proportion of smokers was

1259/2648.

Among subjects who hadbronchitisinthesmoker group, it wasfoundthat the ratio

ofsubjects exposedandnotexposedtohigh air contamination was

168/34.

Meanwhile,amongthe

non-smokergroup,259 subjects wereexposedtohigh air contamination. 8. Create 2x2table for each

stratum/level

basedonthesmokingstatus.

Stratum/level

1=

?

I

__

Total

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13.Isthereadifferenceproportionofsmokersbylevelsofair contamination exposure?

14.Is smoking a confoundingfactortothe relationship betweenair contamination exposureand

bronchitis!

Giveyourreasons. ___

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22 |HENRI Data-driven lesson plans

9. Amongsubjectswhohadbronchitis, howmanywerenon-smokers?

10.Caicuiatethemeasurefor

association/risk

in each

stratum/level

andinterpretthe results.

11.How istherelationshipo?smokingandbronchitis? Total

|

Total

1

12.Issmoking anindependent riskfactor for bronchitis?

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Appendix4.2.Worksheet Task2 on confoundingandinteraction TASK 2

Name:

NoID:

Dataset:

:iie name:CHD.xls(MicrosoftExcelversion 97-2003}

Variable: Alcohol:

1=Alcohol drinker 0=Nonalcohol drinker

Smoking

1= Smoking 0 =Nosmoking

Coronary:

1=Coronaryheart

0 = No coronaryheart

Accordingto the data obtained from a cohortstudy, one of the study objectives isto definethe relationship

:etweenalcohol drinking andrisk ofcoronaryheartdisease,

Usethedataset, and calculate riskestimationforthe relationshipsbelow:

1. Alcohol Drinking withCoronary HeartDisease

CoronaryHeart

Disease

+ Total

Drinking

Alcohol

i

4-|

Total

RR OR=

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2. Alcohol DrinkingwithCoronary HeartDisease amongSmokers (Smoking=l)

CoronaryHeart

Disease

Drinking

Alcohol

3. AlcoholDrinkingwithCoronary Heart Disease amongNon-Smokers(Smoking=0)

CoronaryHeart

Disease

Drinking

Alcohol

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4. SmokingwithCoronaryHeart Disease

CoronaryHeart

Disease

+ Totai

C i,"

smoking

1

+

-Totai

RR 0R=

5. SmokingwithCoronary HeartDisease amongAicohoi Drinkers (A!eohoi=lJ

CoronaryHeart

Disease

+ Total

bmoking

+

-Total

RR 0R=

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6. SmokingwithCoronaryHeartDiseaseamongNon-Alcohol Drinkers(Alcohol=l)

Coronary_Heart

Disease

Smoking

7. Alcohol Drinking withSmoking

CoronaryHeart

Disease

Alcohol

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8. Performadjustedanalysistoseeinfluence ofsmokingon the relationship betweenalcoholdrinkingand

coronaryheartdisease,usingstratifiedanalysis.Calculate ORMH(adjustedj usingthe formula below:

Calculate:

OR..

OR,

Conclusions:

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ISBN17fl-L02-].1414-7-4