20. Descriptive epidemiological studies, cross-sectional studies

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  • Descriptive studies
    • These studies try to identify the relation between a disease and factors like age, gender, occupation, place, time, etc.
    • In descriptive studies no hypothesis is formed and no analysis is performed
  • Case report
    • Description of a case, usually a patient, in a manner which provides information
    • Not a true epidemiological study, as only one case is reported
    • However, many case reports with similar features may raise suspicion of an underlying epidemic
      • This is how AIDS was discovered
    • Example: a report of a patient with a rare disease, or unusual symptoms of a disease
  • Case series
    • A series of similar patient cases
    • Common characteristics between cases can be identified
    • Example: a series of cases of patients with the same disease
    • A register is a complete set of case series for an area
      • Registers provide information of disease burden in a country
  • Cross-sectional studies
    • Can be descriptive or analytical
    • Concurrent (does not look back or forward in time)
      • The exposure and outcome (disease) are measured simultaneously
    • Steps
      • (Randomly) select a study population
      • Divide them into 2 groups depending on whether they currently are exposed to the factor in question
      • Further divide these 2 groups into 2 groups depending on whether they have the disease in question
      • We can now evaluate the association between the factor and the disease
    • Advantages
      • Inexpensive
      • No need to wait for the outcome to happen
    • Disadvantages
      • Gives odds ratio and prevalence, not incidence
      • Very susceptible to bias
      • Difficult to establish a causal relationship with