The content in the slide are derived from books and internets.

1. Case-Control Study

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3. Overview
• Types of Study Design
• Hierarchy of Evidence
• Application of all study design
• Introduction to Case control study
• Features of CCS
• Steps in CCS
• Potential Bias in CC
• Advantages and Disadvantages of CCS
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Revision

4. Major Study Design
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5. Hierarchy of Evidence
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6. Application of different study designs
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7. Introduction
• Aka Case-Referent Study
• Case Control study is the observational epidemiological study of
persons with the disease (or another outcome variable) of interest
and a suitable control group of persons without the disease
(comparison group, reference group)
• It is not correct to call “case-control study” any comparison of a group
of people having a specific outcome with another group free of that
outcome.
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8. By definition, CC study have two population
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9. Introduction
• Cases refer to people who have developed the disease or outcome in
question.
• Controls refer to people who do not have the disease or outcome in
question, and who are selected to estimate the frequency of the
exposure in the population.
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10. Why Case control study is often called as
Retrospective Study????
• Case-control study goes from disease onset backwards to the
postulated causal factors.
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11. Features of Case control Study
• Both exposure and outcome (disease) have occurred before the start
of the study
• The study proceeds backwards from effect (disease) to cause
(exposure)
• It uses a control or comparison group to support or
refute an inference.
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12. Case-Control Vs Cohort
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13. Nested Case control Study Design
• An important type of case-control study in
which cases and controls are drawn from the
population in a fully enumerated cohort.
• Typically, some data on some variables are
already available about both cases and
controls; thus concerns about differential
(biased) misclassification of these variables
can be reduced
• Example: environmental or nutritional
exposures may be analyzed in blood from
cases and controls collected and stored years
before disease onset. A set of controls is
selected from subjects (i.e., non-cases) at risk
of developing the outcome of interest at
the time of occurrence of each case that arises
in the cohort
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14. When is a Case-Control Study Desirable?
• When the disease or outcome being studied is rare.
• When exposure data is difficult or expensive to obtain.
• When the study population is dynamic.
• When little is known about the risk factors for the disease, case-
control studies provide a way of testing associations with multiple
potential risk factor
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15. Steps in Case Control Study
• Selection of Cases and control
• Matching
• Measurement of Exposure
• Analysis and Interpretation
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16. Steps in Case Control Study
• Selection of Cases:
• Based on Case Definition: A set of criteria that must be fulfilled in order to
identify a person as representing a case of a particular disease.
• Involves two specification:
• Diagnostic Criteria:
• Eligibility Criteria: Incident cases are eligible rather than Prevalent cases
• Sources of Cases
• Hospital
• General Population
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17. Steps in Case Control Study
• Selection of Control—crucial process
• Features
• Free from disease
• Must be as similar to the cases as possible, except for the absence of
the disease under study
• As a rule, a comparison group is identified before a study is done,
comprising of persons who have not been exposed to the disease or
some other factor whose influence is being studied.
• Difficulties… if disease exist in subclinical forms where diagnosis isn’t
possible
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18. Steps in Case Control Study
• Sources of Control:
• Hospital control
• Relatives
• Neighborhood control
• General Population
• How many controls are needed?
• If cases are easily available, case: control is 1:1
• If cases are rare and less in number, case: control will be 1: 2 or3 or 4
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19. Steps in Case Control Study
• Matching: Defined as the process of selecting the controls so that
they are similar to the cases in certain characteristics, such as age,
race, sex, socioeconomic status, and occupation
• Types: a) Group Matching b) Individual matching
• Why need matching???
• If more cases than controls are found to have been exposed, we may be left
with the question of whether the observed association could be due to
differences between the cases and controls in factors other than the exposure
being studied.
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20. Steps in Case Control Study
• Group matching (or frequency matching) consists of selecting the
controls in such a manner that the proportion of controls with a
certain characteristic is identical to the proportion of cases with the
same characteristic.
• Example: If 25% of the cases are married, the controls will be selected
so that 25% of that group is also married.
•
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21. Steps in Case Control Study
• Individual Matching: A second type of matching is individual
matching (or matched pairs).
• In this approach, for each case selected for the study, a control is
selected who is similar to the case in terms of the specific variable or
variables of concern.
• For example, if the first case enrolled in our study is a 45-year-old
white woman, we will seek a 45-year-old white female control.
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22. Steps in Case Control Study
• Measurement of Exposure
• Questionnaire
• Interviews
• Lab. Other instrumental investigations
• Studying past record e.g hospital records.
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23. Steps in Case Control Study
• Analysis
• To find out
• Exposure rate among case & control to suspected factor
e.g. rates among exposed & unexposed
• Estimation of disease risk associated with exposure(odds ratio)
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24. Analysis
• Exposure rate among case=50/61
• Exposure rate among control=16/41
• Odds ratio= is the ratio of the odds of
exposure among the cases to the odds
of exposure among the controls
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27. Potential Bias in Case-Control Study
• Control Selection Bias
• Case Selection Bias
• Neyman Bias
• Berksonian Bias
• Self selection Bias
• Information Bias:
• Observer Bias
• Recall Bias
• Confounding
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28. Advantages of case control study
• Easy
• Inexpensive
• Few subjects
• Suitable for rare disease
• No risk to subject
• Study of several different etiological factor
• Risk factor can be identified
• No follow up needed
• Minimal ethical problem
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29. Disadvantages of case control study
• Problem of bias
• Difficulty in selecting control group
• Estimate relative risk only.
• Representativeness of cases & control may be of concerned.
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30. References
• k. park-park's textbook of preventive and social medicine 23rd Edition
• Leon Gordis, 5th Edition
• Miquel Porta-Dictionary-of-epidemiology, 5th Edition
• http://sphweb.bumc.bu.edu/otlt/MPH-Modules/EP/EP713_Case-
Control/EP713_Case-Control_print.html
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31. Thank You