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Case Control Study

This document describes and compares case-control and cohort study designs. A case-control study compares exposures in individuals with a disease (cases) to those without the disease (controls) to identify potential risk factors. A cohort study follows groups of individuals who are exposed or not exposed over time to examine disease outcomes. Case-control studies are more efficient for rare diseases and exposures but can be subject to biases, while cohort studies can better establish temporal relationships but are more time-consuming and expensive. Both study designs aim to measure the association between an exposure and disease using odds ratios or relative risks.

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514 views5 pages

Case Control Study

This document describes and compares case-control and cohort study designs. A case-control study compares exposures in individuals with a disease (cases) to those without the disease (controls) to identify potential risk factors. A cohort study follows groups of individuals who are exposed or not exposed over time to examine disease outcomes. Case-control studies are more efficient for rare diseases and exposures but can be subject to biases, while cohort studies can better establish temporal relationships but are more time-consuming and expensive. Both study designs aim to measure the association between an exposure and disease using odds ratios or relative risks.

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Case control study

 Case group in series of patients who have disease of interest


 Control group or comparison group of individuals without the
disease
 Both the groups are compared
Advantages
 Studying disease with long latency period
 Efficient in terms of time and cost
 For studying rare disease
 Can evaluate a range of causes relating to a specific disease as
well as inter relationship among these factors
Disadvantages
 Biases can occur
 Inefficient in rare exposure
 Temporal relation cannot be established
 Cannot directly compute incidence rate
Cases
 Hospital cohort
 Population based
Controls
 Most critical issue
 Should be comparable to cases
 Comparable to source population
o Hospital control
o General population
o Specific population – friends, relatives
o Case records, death certificate, interviews, mailed
questionnaire, physician record, log book
Bias
 Selection bias – those who participate are different from
individuals who are eligible to participate but are
unwilling or not selected
 Observation bias
Knowledge of the disease status may influence reporting of
information
o Recall bias – cases report more exaggerated
exposure
o Misclassification – error in categorization of
exposure
 Confounding bias
Distortion of study effect mixed with another effect
Because of variables extraneous to exposure- disease
association
Usually occurs when dealing with multiple risk factors whicj
are related to each other
ODDS RATIO
Defined as the odds that a case is exposed divided by the odds
that a control is exposed
 Association of an exposure and disease
 Case control study
Odds ratio= AD/BC
If frequency of exposure is higher among cases, OR >1, indicate
risk
If frequency of exposure is lower among cases, OR <1, indicate
protection
Cohort study
 Analytical study design
 A group of individuals are defined on the basis of presence or
absence of exposure to a suspected risk factor for a disease
 At the time the exposure status is defined, all potential subjects
must be free from the disease under investigation
 The eligible participants are then followed over a period of time
to assess the occurrence of the outcome
Advantage
 Temporal sequence between exposure and disease can be
more clearly established
 Rare exposures. E.g. occupational can be studied
 Can study multiple effects of a single exposure
 Less chance for selection bias
 Incidence rate of disease can be assessed
Disadvantage
 Time consuming and expensive
 Loss to follow up
Types
 Prospective
 Retrospective
Retrospective cohort study:
Both exposure and outcome has already occurred
Prospective cohort study:
Exposure may or may not have occurred
But outcome has not occurred
Nested case control study:
 Modified cohort study
 After a certain number of cases occurred, they are compared
with the controls

MEASURE OF ASSOCIATION
Odds ratio
 Association of an exposure and disease
 Case control study
Odds ratio= AD/BC
If frequency of exposure is higher among cases, OR >1, indicate
risk
If frequency of exposure is lower among cases, OR <1, indicate
protection

Relative risk
 Measure of association of an exposure and disease
 cohort study
 Relative risk= incidence of disease in exposed/ incidence of
disease in unexposed
= A/ A+B/ C/ C+D

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