Introduction to Epidemiologic
Study Design
October 7 2004
Epidemiology 511
W. A. Kukull
Preliminaries
• What is the research question?
– Searching for causal associations between
“exposure” and “disease”
• Describing person, place, time
– Is the exposure common or rare ?
• Can we obtain each individual’s exposure?
• Can we assign the exposure to individuals?
– Is the disease common or rare?
• Duration? Probability of recurrence?
Objectives
• Identify Descriptive vs. Analytic designs
– Recognize appropriate uses
– Describe basic characteristics
• Distinguish between analytic designs
– Describe comparison group formation for
experimental vs. observational designs
– Recognize specific observational designs
Study Designs (1)
Epidemiologic Studies
Major categories
Descriptive Studies
(no hypothesis testing)
Populations
Individuals
Analytic Studies
(hypothesis testing)
Observational;
Experimental
Study Designs (2)
Descriptive Studies
Data From Populations
Correlational
Vital statistics data
Existing data sources
Compare
Different groups
Different times
Data from Individuals
Case reports
Case series
Surveys
Clinical reports of a
single individual
or series of individuals
Prevalence of disease
and exposure at a
point in time
"cross-sectional"
Ecologic study examples
(also known as “correlational”)
• per capita meat consumption and colon
cancer incidence between countries
• geographic variation of aluminum content
in drinking water and incidence of
Alzheimer’s disease
• Stroke mortality and aspirin sales in U.S.
1968 vs. 1988
Ecologic fallacy
• Ecologic studies do not measure individuals
• Average group characteristics are ascribed
• Individuals of most interest may actually
have different exposure
• Disease could be due to other factors
Case reports/Case series
• Pulmonary embolism in 40-yo woman after
beginning OC use
• Profound birth defects in the children of
women who took thalidomide prenatally
• Diagnosis of 5 previously healthy young
men with Pneumocystis pneumonia during
1980
• Toxic shock syndrome
Study Designs (3)
Analytic Studies
Experimental designs
(Randomized,
Controlled Exposure)
Community Trials
Clinical Trials
(Individuals)
Randomization to
Treatments
Observational
Studies
(Uncontrolled exposure
Not Randomized)
Case-Control Studies
Cohort Studies
(Cross-sectional?)
Experimental Studies
• Community Trials
– smoking cessation education programs and per
capita cigarette consumption
• Randomized Controlled Trials
–
–
–
–
–
“the Mercedes” of designs
Minimizes bias and confounding
Randomization; Blinding, placebo controlled
Exposure is administered by investigator
ethical issues , stopping rules
Reference Population
Experimental
Population
Nonparticipants
Participants
(study pop)
R
Treatment “A”
Treatment “B”
Study Designs (4)
Observational Studies
Cohort Studies:
Concurrent and
Retrospective
Case-Control Studies
Choose comparison groups
on the basis of
EXPOSURE
Choose comparison groups
on the basis of
DISEASE
Cohort Studies
Retrospective
Concurrent
Today
Disease
The
future
Prevalent
cases out
Defined Population
EXPOSED
No Disease
The
Past
NOT
EXPOSED
Disease
No Disease
Today
Cohort Studies
• Select subjects based on Exposure
– Exclude “Prevalent” cases
• Follow through time to see who develops
disease
• Compare disease incidence (or mortality)
rate in exposed vs. unexposed
– Relative Risk: primary measure of association
between exposure and disease
“Population” base
Framinghammer City
Non- diseased
Study
enrollees
Loss, death, refusals
before disease develops
Disease cases
Time
Case Control Studies
Hx of
Exposure
No Hx of
Exposure
Cases
(with disease)
Hx of
Exposure
No Hx of
Exposed
Controls
(without disease)
Note: “disease” refers specifically to the disease or condition being studied
Case Control Studies
• Select Cases (persons with disease)
• Select Controls (without disease)
• Determine Exposure History
– Prior to disease onset
• Compare odds of exposure in cases vs. controls
– Odds Ratio: primary measure of association
between exposure and disease; an estimate of the
“relative risk”
Population base??
(after Koepsell & Weiss, 2003)
• If each of the cases had not developed the
disease, would he or she have been included
in the population?
• If each of the non cases in the population
had developed the disease, would he or she
have been included as a case
Nested Case Control
Use data or biomarkers
collected at cohort entry
to determine exposure
status for cases and
controls
Develop
disease
Cases
Cohort study population:
Exposed and Unexposed
Do Not
develop Disease
Sampled or Selected
“Controls”
Nested Case-Control
• Population base of a Cohort study
• Enrolls “cases” that develop from cohort
• Samples or selects “controls” from
remaining at-risk cohort
– minimize recall bias;
– data obtained before disease onset;
– economy
Cross-Sectional Design
Disease and exposure measured simultaneously
Population Survey
Disease and
Exposed
Disease and
No Exposure
No Disease;
Exposure
No Disease;
No Exposure
In a cross-sectional design, you cannot tell whether exposure
happened before disease began, or whether it is a result of disease,
or, whether it is irrelevant to disease etiology.
Conclusion
• A brief tour of study designs
– Descriptive
• Person, Place, Time
• Ecologic studies (descriptive or analytic?)
• Cross-sectional(?)
– Analytic
• Cohort
• Case-Control and “nested” Case-Control
• Randomized controlled trial