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Study Design Cross-sectional Studies

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Study Design Cross-sectional Studies

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    1. Study Design Cross-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

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