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Study Design. Life's a journey. . .The
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1. Study DesignCross-sectional Studies Stephen McCurdy, M.D., M.P.H.
Division of Environmental & Occupational Health
Department of Public Health Sciences
U.C. Davis School of Medicine
3. Study Design Objectives
? ?1. Review the three fundamental study designs
-Cohort (including clinical trials)
-Case-Control
-Cross-Sectional survey
? 2. Discuss Cross-Sectional surveys
-Uses
-Strengths/limitations
4. Study Design
5. Cohort Studies
?Begin with sample ? “Healthy Cohort” (i.e., subjects without the outcome yet)
?Start with Exposure status, then compare subsequent disease experience in exposed vs. unexposed.
6. Study Design
7. Case-Control Studies
? Begin with sample of “Cases and Controls”
? Start with Disease status, then assess and compare Exposures in cases vs. controls.
8. Study Design
9. Cross-Sectional Studies
? Begin with “Cross-sectional” sample
? Determine Exposure and Disease at same time
10. Study Design
11. Cross-Sectional Studies or “PREVALENCE
STUDY”
Hallmark:
Risk factors (exposures) and disease outcome are ascertained at a single point in time in a cross-sectional sample of subjects.
12. Cross-Sectional Studies Uses of Cross-Sectional studies:
Patterns of exposures and outcomes (“the lay of the land”)
?
Burden of illness and risk factors
Service needs
Hypothesis generation
13. Cross-Sectional Studies “How to . . .”
1. Choose population and cross-sectional sample
2. Develop a valid survey instrument
Subjective: Questionnaire
Objective: Exam, Lab Tests
3. Administer Survey
High participation is key!!
14. Sampling frames
Population registries
Censuses
Telephone sampling
Institutions: school, hospital, etc.
Convenience
Other haphazard approaches
Data Collection
Subjective
Objective Cross-Sectional Studies
15. Cross-Sectional Studies Advantages
Quick and cheap
Descriptive role
Examine associations
Disadvantages
Temporal associations not clear
Selection bias
Shows association, not causality
16. Cross-Sectional Studies The selection bias classic for cross-sectional studies is “the healthy worker effect.” I.e., only “healthy workers” are available for study, distorting your findings.
Example: Low asthma rates in animal handlers (because persons contracting asthma quit and are not available for study).
17. Cross-Sectional Studies
18. Cross-Sectional Instead of looking at a ratio of prevalences, we can also look at a ratio of odds.
Odds are not intuitively appealing: they are the likelihood of an event occurring divided by the likelihood of the event not occurring.
19. Cross-Sectional Studies
20. Cross-Sectional Studies
21. Study Design TB infection, shown by a positive skin test, usually remains dormant. However, infected persons should be treated with INH to prevent future TB disease.
Q: How can you determine how big a problem TB infection is among migrant farm workers and if there are any high-risk subgroups?
22. Cross-Sectional Studies An example from the literature:
Tuberculin reactivity among California migrant Hispanic farm workers.
Introduction:
Migrant farm workers at increased risk for TB.
23. Cross-Sectional Studies Methods:
Study population all residents > 1 year of age
Brief health and work questionnaire
Tuberculin skin test (TST) for persons not known to be TST(+)
Follow-up for TST(+) as needed
24. Cross-Sectional Studies Results
Sex
Males - 204 (43.5%)
Females - 265 (56.7%)
Age
27.9 ± 17.1 y ( Mean ± SD)
Place of birth
U.S. - 125 (26.7%)
Non-U.S. - 344 (73.1%)
25. Cross-Sectional Studies
26. Cross-Sectional Studies
27. Cross-Sectional Studies Discussion
Overall prevalence 16.6%
Lower than observed in other settings
Increased prevalence for
Men (OR 1.22)
FS (OR 3.11)
15-39y (OR 2.59)
Non-US Born (OR 2.09)
28. Cross-Sectional Studies Discussion
Increased risk for. . .
– Men: as in other studies
? 15-39 yrs. – not seen in other studies
? FS – not seen in other studies
? Non-US Born – as in other studies
29. Study Design Objectives
?1. Review the three fundamental study designs
-Cohort
-Case-Control
-Cross-Sectional survey
? 2. Discuss Cross-Sectional surveys
-Uses
-Strengths/limitations