Study Designs _Population Sampling

8/21/2019 Study Designs _Population Sampling

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Dr. LakshmiAssistant Professor

(Epidemiology)School of Public HealthP!"E#$ %handigarh

Study designs in Epidemiology


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!nitial &uestions

Appropriate studydesign

Sample si'eSampling

Data collection

Data analysis


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irst Step *b+ecti,e of thestudy-urden of the disease

Hypothesis generationEg /hat is the health seeking

beha,iour of chestsymptomatics in acommunity0

Association

Eg /hat are the risk factors ofdeath after 1- treatment0


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-urden of Disease or Hypothesiseneration %ross2sectional

studiesPro,ide snap shot ofpopulation at a singlepoint of time

E3posure and disease

assessment at thesame point

*utcome "easurementPre,alence

"easure of associationPre,alence ratio orodds ratio


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E3ample#esearch &uestion 1o 4nd out the

pre,alence of chest symptomatics in urbanand rural areas

%onduct the sur,ey in urban and rural areas

Estimate the chest symptomatics in eacharea

!f chest symptomatics are more in rural

area$ 5e can generate hypothesis thatresiding in rural area causes prolongedcough


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%ross sectional Sur,eys %hestSymptomatics$ 67 years and abo,e

Place Sur,eyed

%hestsymptomatics

8

%ity 799 7 6

1o5n 799 :6.;


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Limitations %ross2sectionalstudies

%annot distinguish temporalse=uence of e3posure and disease

A>ected by factors in?uencingpre,alence


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Association AnalyticalStudies%ase control%ohort

!nter,entional studies


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%ase2%ontrol Studies

Controls

No Exposure

Exposure

Cases

Exposure

No Exposure

Assess Select


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%ase %ontrol Study E3ample#esearch &uestion 1o 4nd out the riskfactors for death after 1- treatment%ases Deaths after 1- treatment

%ontrols 1hose sur,i,ed after 1-treatment

E3posure or E3ample$ Smoking

%ompare odds of smoking amongcases and controls by calculating *dds#atio


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*dds of Smoking among Deaths77@6

*dds of Smoking among Sur,i,ors@

DEATHS SURVIVORS

Hypothetical Case Study: Aon! ten deaths o"

T# "ollo$in! treatent% "i&e $ere so'ers(

Aon! )* people $ho sur&i&ed a"ter T#

treatent + $ere so'ers(


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%ase control studies"easure of association *dds ratio

Cases(Deaths)

Controls(Survivors)

Exposed(Smokers)

5(a) 2(b)

NotExposed

(Nonsmokers)

5(c) 8(d)

Odds o

Exposure

5!5(a!c) "# 2!8(b!d)"$%

25*dds #atio @ (ac) (cd) @ 69.;7@B


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%ase control studiesAd,antages/hen aetiology is unkno5nLess time and money

#are diseases and diseases 5ith longlatent periodsDynamic population

Disad,antages

DiCcult to establish temporal se=uence#are e3posuresreater chance of bias


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%ohort studies

Exposed

Unexposed

Diseased

Not Diseased

Diseased

Not Diseased

AssessSelect


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%ohort studies

Prospecti,eE3posed and ne3posedpopulations follo5ed into future for

the de,elopment of outcome#etrospecti,eE3posure and outcome ha,e

already occurred-asic study design is al5ays frome3posure to disease


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E3ample

#esearch &uestion 1o kno5 the risk factors fordeath after treatment for 1-

%ohort All ne5ly diagnosed cases of 1- 5ho5ere kept on D*1S and completed thetreatment and follo5ed to see ho5 many died

after treatmentE3posure or e3ample$ Smoking

*utcome Death after treatment 5ith 1-

Di,ide the cohort into smokers and non smokers

%alculate risk of dying in each group andcompare by using #elati,e risk


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#isk of dying among [email protected]

#isk of dying among onsmokers@

S,O-ERS NON S,O-ERS

Hypothetical Case Study: Aon! ten so'ers

$ho had treatent "or T# . died a"ter treatent

$ith T# $hereas aon! )* non so'ers only +

died


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%ohort studies"easure of association #elati,e risk

Deaths

Survived

&otal

'isk

Exposed(Smokers)

5(a) 5(b) #$(ab)

5!#$"$%5

Not

Exposed(Nonsmokers)

2(c) 8(d) #$

(cd)

2!#$

"$%2

#isk of dying among smokers 769 (aaFb)@ 9.7

#isk of dying among on smokers ;69


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%ohort studiesAd,antages#are e3posuresDirect estimation of riskLess chance of -ias (Prospecti,e)

"ultiple e>ects of a single disease/ell de4ned temporal se=uenceDisad,antages 1ime consuming costly (Prospecti,e)#are outcomes long latent periods

(Prospecti,e)-ias (#etrospecti,e)Poor information on e3posures and

outcomes (#etrospecti,e)


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!nter,ention studies!n,estigator assigns e3posure to the

participants

Study design is from e3posure to outcome

Ethical problems for ha'ardous e3posuresStudied by attempts to eliminate the

e3posures


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!nter,ention studies

Ad,antages"ost reliable e,idence in

epidemiological research becausethe e3posure is randomly assigned#andomisation controls the e>ectsof the risk factors not considered atthe time of the study.


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Sampling


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Sampling "ethods

Simple #andomSampling

Systematic #andomSampling

Strati4ed #andom

Sampling"ulti Stage sampling

%luster sampling


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Simple #andom Sampling

E=ual probability of selection

Sampling frame re=uired

#andom number table%alculator%omputer

9G B BG G :

I B ;B : :; 6: : :; ; ::

6; 7: 7 II ;:

77 7I 7: G7 :B


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Systematic #andomSamplingo sampling frame re=uiredPrecision similar to S#S

1otal population di,ide by sample si'e to getthe sampling inter,al (G99G9@69)

#andom sample of a sampling unit bet5eenthe 4rst unit and the sampling inter,al (6269)

Subse=uent units selected by adding thesampling inter,al in the 4rst random selectedunit$ and then in the second unit and so on.


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Strati4ed SamplingS#S represents strata in the same

proportion as in the populationSeparate estimate may be re=uires for

each stratum (rural$ urban)

Di,ide the sample frame into stratase S#S or systematic random sample to

dra5 sample in each strata/eighted a,erage of the estimate stratum

estimates gi,es population estimates


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"ulti stage Sampling

15o stage4rst stage of administrati,e units (PSs)Second stage of the sampling units in the PS

1hree or multi stage

S#S or Systematic #andom Sampling method usedfor ,arious stage sample units


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Sample si'e

"easurement of interest

Pre,alence$ !ncidence$ "ean$Proportion$ *#$ ##

J K error

#e+ecting ull Hypothesis 5hen it istrue

sually 78 9r 68

K error

ailing to re+ect ull Hypothesis5hen it is false

Po5er@ 62 $ usually 98 9r I98


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Statistical Analysis

-inarySingle Proportion 15o Proportions&uantitati,eSingle "ean 15o "eansDistribution

ormalot ormal


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1est of Signi4cance

"eanSingle M t test 15o M t testPaired Paired t test

"ore than t5o A*

Study Designs _Population Sampling

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