A Brief Introduction to Epidemiology IX Epidemiologic Research Designs: CaseControl Studies

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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 Cross-Sectional 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
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
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Case Control Design

• Time
• Direction of Inquiry

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

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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 -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

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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.
• 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.

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References

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