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Updated: Mar 26 2022

Observational Studies

  • Overview
    • Observational studies are studies in which subjects are observed for the effect of exposures on outcomes without intervention by researchers
    • Often done when randomization is not possible for logistical or ethical reasons
  • Descriptive Studies
    • Case report
      • detailed description of a patient's symptoms, signs, treatment, and disease course
      • conducted for hypothesis generation or to raise awareness of emerging diseases, rare pathologies or unusual presentations
      • not useful for hypothesis testing
    • Case series
      • a group of case reports of patients with a similar exposure, treatment, or disease
      • no comparison group
      • not useful for hypothesis testing
  • Prospective and Retrospective Studies
    • Prospective studies
      • patients enrolled before data collection
      • participants followed over time 
      • data collected on exposures and outcomes as they arise 
    •  Retrospective studies 
      • involve collecting historical information from a sample of patients 
      • enrollment occurs after exposure and outcome have  already occurred
  • Case-Control Study
    • Retrospective (almost always) 
    • Compares patients with and without disease to asses their odds of a certain exposure 
    •  Odds ratio (OR) is measure of disease association
      •  OR = odds of exposure among cases/odds of exposure in non-cases = ad/bc
        • odds of exposure among cases = # cases with exposure/# cases without exposure = a/c
        • odds of exposure among non-cases = # non-cases with exposure/# non-cases without exposure = b/d
    • "How much more likely is it that patients with cirrhosis have been exposed to heavy alcohol use compared to controls?"
  • Cohort Study
    • Can be either prospective or retrospective
      •  In prospective studies, the outcome of interest has not yet occurred and patients are tracked prospectively for development of the outcome
      •  In retrospective studies, the outcome of interest has already occurred 
    • Compares groups with and without an exposure to assess associations with subsequent disease 
    • Relative risk (RR) is measure of disease association 
      • RR= incidence rate in exposed group/incidence rate in unexposed group = [a/(a+b)]/[c/(c+d)]
        • incidence rate in exposed group = # of exposed cases/(# total exposed cases and non-cases) = a/(a+b)
        • incidence rate in unexposed group = # of unexposed cases/(# total unexposed cases and non-cases) = c/(c+d)
    • "How much more likely are patients to develop cirrhosis if they are exposed to heavy alcohol use?"
  • Cross-Sectional Study
    • Exposure and outcome are assessed simultaneously 
    • Ascertains association not causality
    • Prevalence is measure of disease association
    • "How much higher is the proportion of cirrhosis patients with heavy alcohol use compared to those without cirrhosis?"
  • Ecological Study
    • Exposure and outcome assessed at the population level
      • population defined geographically or temporally
    • Prevalence and incidence are measures of disease association
    • Often used for rapid hypothesis generation
    • "How much higher is the proportion of cirrhosis patients in a country where average alcohol use is high compared to a country in which alcohol is illegal and of low use?"
    • Disadvantage is the potential for "ecological fallacy"
      • making spurious risk associations between populations which may not represent true risk association at the individual level
      • e.g., assuming that people who live in a country with high alcohol use rates must be more likely to get cirrhosis by virtue of living in that country
  • Sibling Studies
    • Measures relative influence of environment vs genetics on trait variation
    • Twin concordance study
      • compares how often monozygotic and dizygotic twins both have the same trait or disease
    • Adoption study
      • 2 designs
        • examines differences and similarities between child and adoptive vs birth parents
        • examines differences and similarities between siblings raised by adoptive vs birth parents
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