Exposure - UNC Center for Public Health Preparedness

Report
Session 3, Part 2
Study Designs for Analytic Epidemiology
Learning Objectives
Session 3, Part 2
• Define analytic epidemiology
• List 3 types of observational study designs
Overview
Session 3, Part 2
• Review of descriptive vs. analytic
epidemiology
• Components of analytic epidemiology
• Types of analytic study designs
Descriptive versus Analytic
Epidemiology
Descriptive vs. Analytic
Epidemiology
Questions
Comparison
Group?
Descriptive
epidemiology
• Who
• What
• When
• Where
No
Analytic
epidemiology
• Why
• How
Yes
Analytic Epidemiology
• Used to help identify the cause of disease
• Typically involves designing a study to test
one or more hypotheses
Source: Borgman, J (1997). The Cincinnati Enquirer. King Features Syndicate.
Components of Analytic
Epidemiology
Exposure and Outcome
• Exposure:
Any factor that might influence one’s risk of
disease
• Outcome:
Disease or condition, standardized using case
definitions
Case Definition
• Standard diagnostic criteria that must be
fulfilled to identify a person as a case of a
particular disease
– Clinical (laboratory results, symptoms, signs)
– Restrictions on person, place, and time
• Ensures that all persons who are counted
as cases actually have the same disease
Developing Hypotheses
• A hypothesis is an educated guess about
an association that is testable in a
scientific investigation
• Descriptive data provide information to
develop hypotheses
• Hypotheses tend to be broad initially and
are then refined to have a narrower focus
Hypothesis Example
• Hypothesis: People who ate at the church picnic
were more likely to become ill
– Exposure is eating at the church picnic
– Outcome is illness – this would need to be defined, for
example, ill persons are those who have diarrhea and
fever
Hypothesis Example
• Hypothesis: People who ate at the church picnic
were more likely to become ill
– Exposure is eating at the church picnic
– Outcome is illness – this would need to be defined, for
example, ill persons are those who have diarrhea and
fever
• Hypothesis: People who ate the egg salad at the
church picnic were more likely to have laboratoryconfirmed Salmonella
– Exposure is eating egg salad at the church picnic
– Outcome is laboratory confirmation of Salmonella
Types of Analytic Studies
Main Categories of Studies
• Experimental studies –
exposure status is assigned by
investigators
• Observational studies –
exposure status is not
assigned
Experimental Studies
• Can involve individuals or
communities
• Assignment of exposure status can
be random or non-random
• The non-exposed group can be
untreated, untreated with placebo, or
given a standard treatment
• Most common design is a
randomized clinical trial
Experimental Study Examples
• Randomized clinical trial to
determine if giving magnesium
sulfate to pregnant women in
preterm labor decreases the risk
of their babies developing
cerebral palsy
• Randomized community trial to
determine if fluoridation of the
public water supply decreases
dental cavities
Observational Studies
• Three main study designs:
1. Cross-sectional study
2. Cohort study
3. Case-control study
Cross-Sectional Studies
• Exposure and outcome status are determined at
the same time
– “Snapshot”
• Examples include:
– Behavioral Risk Factor Surveillance System (BRFSS)
http://www.cdc.gov/brfss/
– National Health and Nutrition Surveys (NHANES)
http://www.cdc.gov/nchs/nhanes.htm
• Also include most opinion and political polls
Cohort Study Description
• Study population is grouped by exposure
status
• Groups are then followed to determine if
they develop the outcome
Exposure
Outcome
Prospective
Assessed at
beginning of study
Followed into the
future for outcome
Retrospective
Assessed at some
point in the past
Outcome has
already occurred
Cohort Study Design
Study
Population
Exposure is
self selected
Exposed
Non-exposed
Cohort Study Design
Study
Population
Exposure is
self selected
Non-exposed
Exposed
Follow through
time
Disease
No Disease
Disease
No Disease
Cohort Study Examples
• Study to determine if those exposed to
environmental tobacco smoke (ETS) have a
higher risk of leukemia than those not exposed
to ETS
• Study to determine if children who receive
influenza vaccination miss fewer days of school
than those who did not receive it
• Study to determine if the egg salad was the
cause of a foodborne illness outbreak
Case-Control Study Description
• Study population is grouped by outcome
• Cases are persons who have the outcome
• Controls are persons who do not have the
outcome
• Past exposure status is then determined
Case-Control Study Design
Study
Population
Select based
on disease
status
Cases
Controls
Case-Control Study Design
Study
Population
Select based
on disease
status
Cases
Controls
Look back in
time
Had Exposure
No Exposure
Had Exposure
No Exposure
Case-Control Study Examples
• Study to determine whether women with strokes
had hormone replacement therapy as compared
to women without strokes
• Study to determine whether lung cancer
patients have more radon exposure than nonlung cancer controls
• Study to determine whether salmonella infection
was associated with eating at a fast food
restaurant
Cohort versus Case-Control Study
Cohort Study
Preferred
• Population members
study design
are easily identifiable
when…
• Members are easily
accessible
• Exposure is rare
• There may be multiple
diseases involved
Case-Control Study
• Identifying and/or
accessing entire
cohort would be too
costly or time
consuming
• Illness is rare
Study group
Exposed persons
Persons with illness
(case patients)
Comparison
group
Non-exposed persons
Persons without the
illness (controls)
Summary
• Analytic epidemiology addresses “why” and
“how” a health problem occurs
• In experimental studies investigators assign
exposures to study participants
• In observational studies investigators observe
exposures and outcomes that are already
occurring in the population
• Commonly used observational study designs are
cohort studies and case-control studies
References and Resources
•
•
•
•
•
Centers for Disease Control and Prevention. Principles of Epidemiology.
3rd ed. Atlanta, Ga: Epidemiology Program Office, Public Health Practice
Program Office; 1992.
Gordis L. Epidemiology. 2nd ed. Philadelphia, Pa: WB Saunders Company;
2000.
Gregg MB, ed. Field Epidemiology. 2nd ed. New York, NY: Oxford
University Press; 2002.
Hennekens CH, Buring JE. Epidemiology in Medicine. Philadelphia, Pa:
Lippincott Williams & Wilkins; 1987.
Cohort Studies. ERIC Notebook [serial online]. 1999:1(3). Department of
Epidemiology, University of North Carolina at Chapel Hill School of Public
Health / Epidemiologic Research & Information Center, Veterans
Administration Medical Center. Available at:
http://cphp.sph.unc.edu/trainingpackages/ERIC/issue3.htm. Accessed
March 1, 2012.
References and Resources
•
•
Case-Control Studies. ERIC Notebook [serial online]. 1999:1(5).
Department of Epidemiology, University of North Carolina at Chapel Hill
School of Public Health / Epidemiologic Research & Information Center,
Veterans Administration Medical Center. Available at:
http://cphp.sph.unc.edu/trainingpackages/ERIC/issue5.htm. Accessed
March 1, 2012.
Laboratory Instructor’s Guide: Analytic Study Designs. EPID 168 Lecture
Series. Department of Epidemiology, University of North Carolina at Chapel
Hill School of Public Health; August 2002. Available at:
http://www.epidemiolog.net/epid168/labs/AnalyticStudExerInstGuid2000.pdf
. Accessed March 1, 2012.

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