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Evidence-Based Medicine 2

Evidence-Based Medicine 2. More Knowledge and Skills for Critical Reading Karen Schetzina, MD, MPH. Purposes of Epidemiologic Studies. Identify new diseases Identify populations at risk for a disease Identify possible causative agents of a disease

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Evidence-Based Medicine 2

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  1. Evidence-Based Medicine 2 More Knowledge and Skills for Critical Reading Karen Schetzina, MD, MPH

  2. Purposes of Epidemiologic Studies • Identify new diseases • Identify populations at risk for a disease • Identify possible causative agents of a disease • Identify factors or behaviors that increase risk of disease and their relative importance

  3. Purposes of Epidemiologic Studies (continued) • Rule out factors or behaviors as contributing to a disease • Evaluate therapies for a disease • Guide in the development of effective public health measures and preventive strategies

  4. Reading Epidemiologic Studies • Become familiar with medical terminology (but you don’t need to know every word) • Have some prior knowledge of the pathophysiology of the disease of interest • Understand principles of study design, analysis, and interpretation

  5. Review - Definitions • Hypothesis or research question • Population – Collection of units from which a sample may be drawn • Sample – A selected subset of a population • Variable - “Any quantity that varies. Any attribute, phenomenon, or event that can have different values.” • Predictor Variable – Risk factor • Outcome Variable – Disease or condition of interest • Association or Correlation – Degree to which variables change together

  6. Review - Definitions • Estimate – Incorporates some degree of error • Prevalence - “The number of events in a given population at a designated time” • Incidence – “The number of new events in a defined population within a specified period of time” • Mean – average • Variance – A measure of dispersion or variation • Standard Deviation/Error – Square root of the variance

  7. Strength Consistency Specificity Temporality Biologic gradient Plausibility Coherence Experimental evidence Analogy Hill’s Causal Criteria

  8. Study Designs • Observational, Descriptive • Observational, Analytic • Experimental

  9. Study Designs • Observational, Descriptive • Describe the amount and distribution of disease • Useful for generating hypotheses • Observational, Analytic • Designed to test one or more specific hypotheses • Experimental • The exposure and the persons to be exposed are determined by the investigator

  10. Study Designs • Observational, descriptive • Case reports, case series • Ecological • Cross-sectional • Observational, analytical • Case control • Cohort • Experimental • Randomized clinical trial • Community Intervention Trial

  11. Case report or series • Describe patients or a series of patients with an interesting presentation of disease

  12. Ecological Studies • Studies correlating two or more characteristics of populations – unit of analysis is the group, not the individual • Per capita consumption of cigarettes correlates with national mortality from lung cancer in countries throughout the world.

  13. Ecological Studies • Advantages • Quick, easy, and cost-effective • Disadvantages • Cannot be extrapolated to individuals • Correlations can be misleading

  14. Cross-Sectional Study • Measures exposure to suspected risk factors and the presence of a disease or condition in people in a population at the same time • In a school survey, children who had disciplinary problems were found to spend more hours in front of the television at home.

  15. Cross-Sectional Study • Advantages • Relatively quick and inexpensive • Disadvantages • Difficult to assess temporal associations between risk factor and disease (tough to differentiate cause and effect) • See over-representation of diseases of long duration (because measuring prevalence)

  16. Study Designs • Observational, descriptive • Case reports, case series • Ecological • Cross-sectional • Observational, analytical • Case control • Cohort • Experimental • Randomized clinical trial • Community Intervention Trial

  17. Case Control Study • People with and without a disease or condition of interest are identified and their prior exposure to suspected risk factors is measured. • The proportion of smokers among lung cancer patients admitted to a hospital was found to be much higher than the proportion of smokers among patients admitted for other reasons

  18. Source:Bambang Sutrisna, MD, MHS, DrPH, University of Indonedia, http://www.pitt.edu/~super1/index.htm

  19. Case Control Study • Advantages • Relatively quick and inexpensive • Good for rare diseases • Disadvantages • Information on suspected risk factors may be incomplete • Potential problem of recall bias • Selection of an appropriate control group can be difficult

  20. Cohort Study • People with and without exposure to a suspected risk factor are identified and followed to determine whether they develop the disease(s) or condition(s) of interest. • In the Framingham Heart Study, a cohort of disease-free individuals were evaluated for blood pressure levels, cholesterol levels, and other characteristics, and followed for over 40 years for the development of coronary heart disease.

  21. Cohort Study

  22. Cohort Study • Advantages • Can better establish temporality (because measure the risk factor before the disease is diagnosed) • Can get incidence rates • Can study many diseases • Disadvantages • Take a long time to complete, require large sample size, and are expensive • Loss-to-follow-up bias may occur • Inefficient for studying rare diseases

  23. Analysis of Results

  24. Study Designs • Observational, descriptive • Case reports, case series • Ecological • Cross-sectional • Observational, analytical • Case control • Cohort • Experimental • Randomized clinical trial • Community Intervention Trial

  25. Experimental Study • Basically a prospective cohort study in which the exposure and the persons to be exposed are determined by the investigator. The randomized controlled trial is the strongest method to determine cause and effect in human subjects.

  26. Source:Bambang Sutrisna, MD, MHS, DrPH, University of Indonedia, http://www.pitt.edu/~super1/index.htm

  27. Source:Bambang Sutrisna, MD, MHS, DrPH, University of Indonedia, http://www.pitt.edu/~super1/index.htm

  28. Is having a regular source of medical care important for lowering the risk of cardiovascular disease among Mexican Americans? • Introduction • Study Design • Population of Interest • Prediction & Outcome Variables • Criteria for Inclusion of Articles • Summary of Findings • Strengths & Limitations • Conclusions & Future Research

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