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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