3. CROSSSECTIONAL & ECOLOGIC

3. CROSSSECTIONAL & ECOLOGIC

1. CROSS SECTIONAL

   • population base with disease assessed simultaneous with exposure--is retrospective
     • 1. Define study population
       • 2. Researchers establecen prevalence
         • prevalence ratio & prevalence odds ratio
           • INFLUENCED by INCIDENCE RATE & DURATION of disease (longer disease or more new cases, numerator goes up)
             • ANTECEDENT-CONSEQUENCE BIAS (que fue primero, exposure or outcome)

2. ECOLOGIC STUDIES

   • unit of measurement: GROUP LEVEL
     • LINKING INDIVIDUAL EXPOSURE is a critical difference
       • ECOLOGIC FALLACY: concluding that an association between the exposure and the health outcome at a group level is true at an individual level, when this may not be true.

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