3. case control study

7/29/2019 3. Case Control Study

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Dr. Maher D. Fuad Fuad

Lecturer

Faculty of Medicine

Department of Community Medicine


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Definition

Observational epidemiologic study

Persons with the disease & a suitable control(comparison, reference) group of persons without thedisease.

The relationship of an attribute to the disease is

examined by comparing the diseased and non-diseased with regard to how frequently the attribute ispresent


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Def. continue It is a retrospective or backward looking study as the

study commences with the cases in which disease inquestion had already appeared.


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Purpose

Establish the cause and effect relationship (causal

association) Testing causal hypothesis for rare diseases

When results are expected less expensively and inshort time


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EXPOSED

EXPOSED

NOT

EXPOSED

NOTEXPOSED

TIME

Direction of inquiry


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STEPS OF CASE CONTROL STUDY Selection of cases

Selection of controls

Matching Enquiry and records verification for amount of

exposure in both groups

Comparison analysis and risk measurement

If exposure is more in cases than in controls causalassociation suspected


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Selection of Cases Study begins with CASES- Patients in whom the disease has already occurred- were enquired for all the details of their exposure to thesuspected cause

Do the cases need to represent all patients with thedisease? No Restricted to a specific type or severity Usually new cases (incidence) is chosen not prevalent

cases? Similar clinical, histological, pathological. Duration of exposureSelected independent ofexposure. (very important recap

XS)


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Continue.. Time of admission and duration of hospital stay

has also to be considered while selecting thecases.

Hospital cases will be usually of advanced stage ofdisease and this factor should be kept in mindwhile selecting the cases

otherwise it will lead to selection bias (error).

If possible, it is always advisable to select all cases

from similar environmental exposure. Sources

Hospital

Community


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Selection of Controls

Carefully chosen otherwise validity of the study will be

defective Should be obtained from same population

Hospital-based.

Community-based.

Friends, relatives, Neighbors.


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Hospital-basedADVANTAGES DISADVANTAGES

Efficiency in obtainingsubjects.

Can carry out Diagnostictest.

Less recall bias

Willingness to participate.

Less likely to drop out.

Cost saving

Unknown potentialconfounders drugs.

Exclusion criteria=case

Without exclusion riskunderestimate


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Community-basedADVANTAGES DISADVANTAGES

Prevalence of exposure can beused as an estimate ofexposure

Sampling frame Cost contacting lists of

potential controls

Interviewing subjects


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Friends, Relatives, NeighborsADVANTAGES DISADVANTAGES

Cooperation

Predominantly healthy Can adjust confounders by

cultural or geographical

Over matching underestimate true effect


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Selection ofControlscontinue..

Number of controls taken may be

four timesTwice

even equal to that of cases

the ratio depends upon the availability of suitable

matched controls, available time and finances.

Equal number, if they are better matched, are enoughto conduct the study.


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MATCHING Is a comparative technique of neutralizing all other

variables present in cases and control EXCEPT thevariable under study

Known confounding variable eg. age, gender,occupation etc.

Unknown confounding variable playing a role in

causation


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Cases are made identical with controls i.e. all theknown variables of the cases (like age, sex, occupation,social status,..etc) are (eliminated) with those ofcontrols except the factor (disease) under study.

Cases and controls of same age, same sex, sameoccupation and living same village may be selected forbetter comparison, if feasible.


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When selected from hospitals, it is essential to choosepatients with other diseases, not with the diseaseunder question and if possible, preferably from same

hospital from where the cases were selectedStudy on the association between lung cancer andsmoking

if lung cancer cases were selected as study group

thyroid cancer patients can be taken as controls fromthe same hospital.

This improves the efficiency of comparison and alsothe validity of the study by avoiding error


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Both the groups were made identical in all their other

characteristics except for the presence of lung cancer,which will be present only in cases.

This can be done by proper selection and by matchingthe known variables in both groups.

Then, we measure the proportion of smoking in casesand controls and compare.

If it is significantly more in cases than in controlgroup, causal association is suspected


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Design of case-control studiesFirst select

Then measure pastexposure

Cases with disease Control withoutdisease

Exposed a b

Not exposed c d

a+c b+d

Proportions Exposed P E/D+ = [a/(a+c)] P E/D- = [b/(b+d)]

Ratio between above 2 proportions is mathematically possible but conceptuallywrong because they are not true prevalences


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Hypothetical example of case control

study of CHD and Cigarette SmokingCHD CONTROL

SMOKE 112 176

DO NOT SMOKE 88 224

TOTAL 200 400

OR = ??????


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What should be always true for a

case-control study?1. Cases and controls are randomly selected.

2. Cases are a representative sample of all cases in thegeneral population

3. Controls are a representative sample of the generalpopulation

4. Cases and controls have the same population of

origin5. Always start with some cases, then identify their

valid controls


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RR & AR in case-control study? Recall WHAT is Relative Risk & Attributable Risk?

RRduration = Risk (E+)/ Risk (E-)

ARduration= Risk (E+

) Risk (E-

) Since risk cannot be computed directly from a case-

control study (why?), RR and AR cannot be computed.

Answer: because we can not calculate incidence.


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Advantages of Case-Control

Studies1. Less expensive and simpler

2. Require smaller sample sizes

3. Shorter duration than prospective study4. Study multiple risk factors for 1 disease.

5. Easily reproduced in different populations bydifferent investigators.

6. Used for rare diseases.


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Disadvantages of Case-Control

Studies1. Liable for selection and recall bias.

2. Population of origin for cases is difficult to defineprecisely.

3. Difficult to identify appropriate control group

4. Does not provide estimate of risks and attributablerisk.

5. not suitable for rare exposures


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Thank you

3. case control study

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