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A Brief Introduction to Epidemiology - IX (Epidemiologic Research Designs: Case-Control Studies)

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Most epidemiological studies are observational. Epidemiological Study Designs Observational Studies ... A case-control study starts with the selection of cases. – PowerPoint PPT presentation

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Title: A Brief Introduction to Epidemiology - IX (Epidemiologic Research Designs: Case-Control Studies)


1
A Brief Introduction to Epidemiology -
IX (Epidemiologic Research Designs Case-Control
Studies)
  • Betty C. Jung, RN, MPH, CHES

2
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

3
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.

4
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)

5
Research Designs in Analytic Epidemiology
  • Ecologic Designs Cross-Sectional Study
  • Case-Control Study
  • Cohort Study

6
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

7
Examples
  • The relationship between thalidomide and unusual
    limb defects in Germany
  • The relationship between meat consumption and
    enteritis necroticans in Papua New Guinea

8
Value
  • Simple to conduct
  • Cost-effective way to study a rare disease

9
Case-Control Studies Methodology
First Select the Cases and Controls
Cases
Control (without disease)
(with
disease)
Then measure, post-exposure
Were Exposed
A
B
D
C
Not Exposed
AC
BD
Population Exposed
A/AC
B./BD
10
Case Control Design
  • Time
  • Direction of Inquiry

Cases with the Disease
Exposed
Population
Not Exposed
Controls without the disease
Exposed
Not Exposed
11
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

12
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

13
Strength of Association
  • Relative Risk(Prevalence) Odds Ratio
    Strength of Association
  • 0.83-1.00 1.0-1.2 None
  • 0.67-0.83 1.2-1.5 Weak
  • 0.33-0.67 1.5-3.0 Moderate
  • 0.10-0.33 3.0-10.00
    Strong
  • lt0.01 gt10.0 Approaching

    Infinity

14
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.

15
Methodological Weaknesses
  • Biased reporting of the antecedent (having lung
    CA -gt 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

16
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

17
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

18
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.
  • Berksons 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.

19
References
  • For Internet Resources on the topics covered in
    this lecture, check out my Web site
  • http//www.bettycjung.net/
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