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.