1. Case controlCase control
studystudy

2. IntroductionIntroduction
 It is analytical studyIt is analytical study
 First approach to test the etiological hypothesisFirst approach to test the etiological hypothesis
 Relatively simple and economical to carry outRelatively simple and economical to carry out
and increasingly used to investigate causes ofand increasingly used to investigate causes of
diseases, especially rare diseases.diseases, especially rare diseases.

3. Characteristics of case-control studyCharacteristics of case-control study
 Both exposure and outcome occurred before theBoth exposure and outcome occurred before the
study.study.
 Study backward from effect to cause.Study backward from effect to cause.
 Use of control group to support an inference.Use of control group to support an inference.

4. Pictorial design of case controlPictorial design of case control
studystudy
 ..
Exposed
Not exposed
Exposed
Not exposed
Case
Control

5. Pictorial design of case controlPictorial design of case control
studystudy
Status ofStatus of
exposedexposed
CaseCase ControlControl
SmokerSmoker 3333 4343
Non-SmokerNon-Smoker 22 2727

6. Steps of case-control studySteps of case-control study
1.1. Selection of case and controlSelection of case and control
2.2. MatchingMatching
3.3. Measurement of exposureMeasurement of exposure
4.4. Analysis /InterpretationAnalysis /Interpretation

7. 1.Selection of case an control1.Selection of case an control
 Selection of casesSelection of cases
– Defining casesDefining cases
Diagnostic CriteriaDiagnostic Criteria
EligibilityEligibility
– Sources of casesSources of cases
HospitalHospital
General PopulationGeneral Population
 Selection ofSelection of
controlcontrol
– HospitalsHospitals
– RelativesRelatives
– NeighborhoodNeighborhood
– General populationGeneral population

8. 2.Matching2.Matching
 Matching is the process by which we selectMatching is the process by which we select
controls in such a way that they are similar incontrols in such a way that they are similar in
case with regard to certain pertinent selectedcase with regard to certain pertinent selected
variables, which are known to influence thevariables, which are known to influence the
outcomes of the diseasesoutcomes of the diseases and which if notand which if not
adequately matched for comparability couldadequately matched for comparability could
distort or confound the result.distort or confound the result.
 This step is very important for case-controlThis step is very important for case-control
study.study.

9. 3.Measurement of exposure3.Measurement of exposure
InterviewsInterviews
Hospital recordsHospital records
Past recordsPast records
Family InterviewFamily Interview
Personal case study/DiaryPersonal case study/Diary

10. 4.Analysis4.Analysis
 Analysis should be done asAnalysis should be done as
– Exposure rateExposure rate
Exposures among casesExposures among cases
Exposures among controlsExposures among controls
–Estimation of riskEstimation of risk
Estimation of Relative riskEstimation of Relative risk
Odds ratioOdds ratio

11. Estimation of riskEstimation of risk
 Estimated relative risk- It is usually notEstimated relative risk- It is usually not
calculated in case-control study. The risk ratio orcalculated in case-control study. The risk ratio or
relative risk is the ratio of occurrence of arelative risk is the ratio of occurrence of a
diseases among exposed people to that non-diseases among exposed people to that non-
exposed people.exposed people.
RR -RR - IncidenceamongcasesIncidenceamongcases
IncidenceamongcontrolsIncidenceamongcontrols

12. Estimation of riskEstimation of risk
Odds ratio is the measurement of strength of theOdds ratio is the measurement of strength of the
association between risk factor and outcome. It is crossassociation between risk factor and outcome. It is cross
product of the case control data table.product of the case control data table.
Odds ratio based on three assumptions,Odds ratio based on three assumptions,
– The diseases must be relatively rare.The diseases must be relatively rare.
– Case represent the diseases.Case represent the diseases.
– Control represent without the diseases.Control represent without the diseases.
 Odds ratio= ad/bcOdds ratio= ad/bc

13. Calculation ?Calculation ?
Status of exposedStatus of exposed CaseCase ControlControl
SmokerSmoker 3333 4343
Non-SmokerNon-Smoker 22 2727
Exposures among cases
Exposures among controls
Estimation of risk
Estimated Relative risk
Odds ratio

14. Bias in case control studyBias in case control study
 Bias due to confoundingBias due to confounding
 Memory or recall biasMemory or recall bias
 Selection biasSelection bias
 Berkesonian biasBerkesonian bias
 Interviewer biasInterviewer bias
 Information BiasInformation Bias

15. Advantages of case-control studyAdvantages of case-control study
 Easy to studyEasy to study
 Rapid and InexpensiveRapid and Inexpensive
 Suitable to study for rare diseasesSuitable to study for rare diseases
 Risk factor can be identified.Risk factor can be identified.
 Study several etiological factor in single time.Study several etiological factor in single time.
 Ethical problem minimal.Ethical problem minimal.

16. DisadvantagesDisadvantages
 Problem of biasProblem of bias
 Selection of control is difficult.Selection of control is difficult.
 Incidence and relative risk cannot be measured.Incidence and relative risk cannot be measured.
 Not suitable for evaluation of therapy.Not suitable for evaluation of therapy.
 Do not distinguish between risk factor andDo not distinguish between risk factor and
associated factors.associated factors.

17. Thank YouThank You