Managerial Epidemiology
Chapter 6
Study Designs: Ecologic, Cross-Sectional, Case-Control
Learning Objectives
Define the basic differences between observational and experimental epidemiology
Identify an epidemiologic study design by its description
List the main characteristics, advantages, and disadvantages of ecologic, cross-sectional, and case-control studies
Describe sample designs used in epidemiologic research
Calculate and interpret an odds ratio
How Study Designs Differ
Number of observations made
Directionality of exposure
Data collection methods
Timing of data collection
Unit of observation
Availability of subjects
Observational vs. Experimental Approaches
Manipulation of study factor
Was exposure of interest controlled by investigator?
Randomization of study subjects
Was there use of a random process to determine exposure of study subjects?
Typology of Epidemiologic Research
Overview of Study Designs
Experimental studies
Quasi-experimental studies
Observational studies
Descriptive studies: cross-sectional surveys
Analytic studies: many ecologic studies, case-control studies, cohort studies
The 2 by 2 Table Represents the Association Between Exposure and Disease Status
Ecologic Studies
The unit of analysis is the group, not the individual.
They can be used for generating hypotheses.
The level of exposure for each individual in the unit being studied is unknown.
Generally makes use of secondary data.
Advantageous with cost and duration.
Types of Ecologic Studies
Ecologic comparison study—involves an assessment of the correlation between exposure rates and disease rates among different groups over the same time period.
Ecologic trend study—involves correlation of changes in exposure with changes in disease within the same community, country, or other aggregate unit.
Example of an Ecologic Correlation
The association between breast cancer and dietary fat for 39 countries.
High intakes of dietary fats associated with high rates of breast cancer mortality.
Examples of Questions Investigated by Ecologic Studies
Is the ranking of cities by air pollution levels associated with the ranking of cities by mortality from cardiovascular disease, adjusting for differences in average age, percent of the population below poverty level, and occupational structure?
What are long-term trends (1950-1995) for mortality from the major cancers in the US, Canada, and Mexico?
Applications of Ecologic Approach
The effect of fluoridation of the water supply on hip fractures
The association of naturally occurring fluoride levels and cancer incidence rates
The relationship between neighborhood or local area social characteristics and health outcomes
The Ecologic Fallacy: Definition
Observations made at the group level may not represent the exposure-disease relationship at the individual level.
The ecologic fallacy occurs when incorrect inferences about the individual are made from group level data.
Implications of the Ecologic Fallacy
The conclusions obtained from an ecologic study may be the reverse of those from a study that collects data on individual subjects.
The Ecologic Study: Example
An ecologic study examines 10 individuals who go into the sun.
The study finds that 7 persons (70%) have sunburned foreheads although 6 persons (60%) wore hats.
The expected number of sunburned foreheads is 4 (the number who did not wear hats).
The media report that wearing hats will not protect you from sunburn.
What the Individual Data Show
Individual Data (cont’d)
From the individual data, one observes that 100% of persons (4) who did not wear hats were sunburned.
Among persons who wore hats (6), only 50% were sunburned.
This conclusion reverses the conclusion from the ecologic data, i.e., that wearing hats affords little protection from sunburn.
Ecologic Studies: Advantages and Disadvantages
Advantages
Quick, simple, inexpensive
Good approach for generating hypotheses when a disease is of unknown etiology
Disadvantages
Ecological fallacy
Imprecise measurement of exposure and disease
Cross-Sectional Study
Also termed prevalence study
Exposure and disease measures obtained at the individual level.
Single period of observation
Exposure and disease histories are collected simultaneously.
Both probability and non-probability sampling is used.
Cross-Sectional Study: Examples
Surveys of smokeless tobacco use among high school students
Prevalence surveys of the number of vasectomies performed
Prevalence surveys of cigarette smoking among Cambodian Americans in Long Beach, California
Uses of Cross-Sectional Studies
Hypothesis generation
Intervention planning
Planning health services and administering medical care facilities
Estimation of the magnitude and distribution of a health problem
Examine trends in disease or risk factors that can vary over time
Limitations of Cross-Sectional Studies
Limited usefulness for inferring disease etiology
Do not provide incidence data
Cannot study low prevalence diseases
Cannot determine temporality of exposure and disease
Overview of Case-Control Studies
In a case-control study with two groups, one group has the disease of interest (cases) and a comparable group is free from the disease (controls).
The case-control study identifies possible causes of disease by finding out how the two groups differ with respect to exposure to some factor.
Characteristics of the Case-Control Study
A single point of observation
Unit of observation and the unit of analysis are the individual
Exposure is determined retrospectively
Does not directly provide incidence data
Data collection typically involves a combination of both primary and secondary sources.
Selection of Cases
Two tasks are involved in case selection:
Defining a case conceptually
Identifying a case operationally
Sources of Cases
Need to define a case conceptually
Ideally, identify and enroll all incident cases in a defined population in a specified time period
A tumor registry or vital statistics bureau may provide a complete listing of all cases
Medical facilities also may be a source of cases, but not always incident cases
Selection of Controls
The ideal controls should have the same characteristics as the cases (except for the exposure of interest).
If the controls were equal to the cases in all respects other than disease and the hypothesized risk factor, one would be in a stronger position to ascribe differences in disease status to the exposure of interest.
Sources of Controls
Population-based controls--Obtain a list that contains names and addresses of most residents in the same geographic area as the cases.
A driver’s license list would include most people between the ages of 16 and 65.
Tax lists, voting lists, and telephone directories
Patients from the same hospital as the cases
Relatives of cases
Measures of Association Used in Case-Control Studies
Case-Control Studies Sample Calculation
On the association between chili pepper consumption and gastric cancer risk: a population-based case-control study conducted in Mexico City
Source: Lopez-Carillo, et al. Am J Epidemiol. 1994;139:263-71.
Sample Calculation (cont’d)
| Chili Pepper Consumption | Cases of Gastric Cancer | Controls |
| Yes | A = 204 | B = 552 |
| No | C = 9 | D = 145 |
The OR (unadjusted for age and sex) is:
AD = (204)(145) = 5.95
BC (552)(9)
Interpretation of an Odds Ratio (OR)
OR = 1 implies no association.
Assuming statistical significance:
OR = 2 suggests cases were twice as likely as controls to be exposed.
OR<1 suggests a protective factor.
Odds Ratio (cont’d)
An OR provides a good approximation of risk when:
Controls are representative of a target population.
Cases are representative of all cases.
The frequency of disease in the population is small.
Examples of Case-Control Studies
Young women’s cancers resulting from utero exposure to diethylstilbestrol
Green tea consumption and lung cancer
Maternal anesthesia and development of fetal birth defects
Passive smoking at home and risk of acute myocardial infarction
Household antibiotic use and antibiotic resistant pneumococcal infection
Advantages of Case-Control Studies
Tend to use smaller sample sizes than surveys or prospective studies
Quick and easy to complete
Cost effective
Useful for studies of rare diseases
Limitations of Case-Control Studies
Unclear temporal relationships between exposures and diseases
Use of indirect estimate of risk
Representativeness of cases and controls often unknown
Key Points to Remember
Descriptive studies: cross-sectional surveys (hypothesis generation)
Analytic studies: ecologic, case-control, and cohort (hypothesis testing)
Conclusion
Study designs differ in a number of key respects, including the unit of observation; the unit of analysis; the timing of exposure data in relation to occurrence of disease endpoint; complexity; rigor; and amount of resources required.
Disease Status
Yes (Cases)
No (Controls)
Yes
A
B
Exposure
Status
No
C
D
A+C B+D
Odds A/C B/D
Odds Ratio AD/BC
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Disease Status |
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Yes (Cases) |
No (Controls) |
ExposureStatus |
Yes |
A |
B |
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No |
C |
D |
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A+C |
B+D |
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Odds |
A/C |
B/D |
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Odds Ratio |
AD/BC |
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