A Brief Introduction to
Epidemiology - IX
(Epidemiologic Research
Designs: Case-Control Studies)
Betty C. Jung, RN, MPH, CHES
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Learning/Performance Objectives
To
develop an understanding of:
– What case-control studies are
– The value of such studies
– The basic methodology
– Pros and Cons of such studies
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Introduction
Epidemiology studies the distribution
of disease in a number of ways.
The two major categories of
epidemiological studies are:
Observational and experimental
studies.
Most epidemiological studies are
observational.
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Epidemiological Study Designs
 Observational
Studies - examine
associations between risk factors and
outcomes (Analytical - determinants and
risk of disease, and descriptive - patterns
and frequency of disease)
 Intervention Studies - explore the
association between interventions and
outcomes. (Experimental studies or
clinical trials)
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Research Designs in
Analytic Epidemiology
Ecologic
Designs CrossSectional Study
Case-Control Study
Cohort Study
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Case-Control Studies
“Flashback
Studies” (Paffenbarger, 1988)
Retrospective - compare cases and controls
for presence of disease
Includes passage of time.
Historical - assess past characteristics or
exposures in two groups of people- cases
and controls
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Examples
The relationship between
thalidomide and unusual limb
defects in Germany
The relationship between meat
consumption and enteritis
necroticans in Papua New Guinea

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Value
Simple
to conduct
Cost-effective way to study a
rare disease
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Case-Control Studies: Methodology
First Select the Cases
and Controls
Then measure,
post-exposure
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Cases
(with
disease)
Control
(without disease)
Were Exposed
A
B
Not Exposed
C
D
A+C
B+D
A/A+C
B./B+D
Population
Exposed
Case Control Design
Time
Direction of Inquiry
Exposed
Not Exposed
Exposed
Not Exposed
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Cases with
the Disease
Controls without
the disease
Population
Case-Control Studies
Cases
- Has condition or health outcome
of interest. Has higher frequency or
greater degree of exposure than non-cases.
Controls (non-cases) - Does not have the
health condition. Serves as the comparison
group
Ask about history of contact with or
exposure to supposed causes
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Case-Control Studies
If
controls are well chosen, the only
antecedent difference will be in the
level of a characteristic that is
related causally to the development
of a disease (I.e, exposure to a
chemical resulted in cancer).
Quantify with odds ratios
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Strength of Association
Relative Risk;(Prevalence); Odds Ratio
0.83-1.00
0.67-0.83
0.33-0.67
0.10-0.33
<0.01
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1.0-1.2
1.2-1.5
1.5-3.0
3.0-10.00
>10.0
Strength of Association
None
Weak
Moderate
Strong
Approaching
Infinity
Methodology Issue: Matching
 Matching
- control for confounding variables. If
you do not match then control by subject selection
(study only males to eliminate gender as a
confounding variable)
 Matching
– Subject selection
– Statistical control during data analysis
– The more variables that need to be matched the
greater the universe we need.
– Problem - match age, sex and SES - Control
must be the same.
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Methodological Weaknesses
Biased
reporting of the antecedent
(having lung CA -> patients over
reporting smoking (from guilt, knowledge
or selective memory)
Subject selection (decreases with cases
and controls in the same facility)
Limited to only cases, who have survived
at the same time. Selective survival
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Pros
 Cases
easily available
 Good for less common or rare cases
 Quick, inexpensive
 Can be conducted by clinicians in clinical facilities
 Tend to support, not prove causal hypothesis by
establishing associations
 Historical data available in clinical records
 Number of subjects needed is small
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Cons
Info
about antecedents depends on
memory, which could lead to bias
Clinical data may be inadequate or
incomplete
“Case group” may not be
homogenous - criteria for diagnosis
may differ.
Clinical cases are selective survivors
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Cons
 Non-representativeness
of cases. Those coming in for
treatment may differ from those not seeking
treatment and those going somewhere else.
 Antecedent is not obtained from universe of all
antecedents.
 Berkson’s fallacy - making generalizations from
hospital or clinical samples to the general population.
 Cannot know what association would be for all or for
a representative sample of all people having the
antecedent.
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References
For
Internet Resources on the
topics covered in this lecture,
check out my Web site:
http://www.bettycjung.net/
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