study designs for analytic epidemiology session 3, part 2
TRANSCRIPT
Study Designs for Analytic Epidemiology
Session 3, Part 2
Learning ObjectivesSession 3, Part 2
• Define analytic epidemiology
• List 3 types of observational study designs
OverviewSession 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
Descriptive epidemiology
Analytic epidemiology
Questions • Who• What• When • Where
• Why• How
Comparison Group? No 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 laboratory-confirmed 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
StudyPopulation
Exposed Non-exposed
Exposure isself selected
Cohort Study Design
Disease No Disease
StudyPopulation
Exposed Non-exposed
No DiseaseDisease
Exposure isself selected
Follow throughtime
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
StudyPopulation
Cases Controls
Select based on disease
status
Case-Control Study Design
Had Exposure No Exposure
StudyPopulation
Cases Controls
No ExposureHad Exposure
Select based on disease
status
Look back intime
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 non-lung cancer controls
• Study to determine whether salmonella infection was associated with eating at a fast food restaurant
Cohort versus Case-Control Study
Cohort Study Case-Control Study
Preferred study design when…
• Population members are easily identifiable
• Members are easily accessible
• Exposure is rare
• There may be multiple diseases involved
• 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.