Commed

• Association


  The concurrence of two variables more often than would be expected by chance
• Epidemiological studies


  • Determine various associations between an exposure and a disease
  • Determine whether the exposure is causal for the disease or not
• Types of association


  • Spurious association
  • Indirect association
  • Direct (causal) association
  • One-to-one causal association
  • Multifactorial association
• Spurious association


  Association between a disease and suspected factor may not be real
• Indirect association


  Associations which at first appeared to be causal have been found on further study to be due to a confounding variable
• Indirect association


  • High altitude
  • Goiter
• Direct (causal) association


  Two variables (A and B) are stated to be causally related if a change in A is followed by a change in B
• Direct (causal) association


  • Measles virus
  • Measles
• Multifactorial association


  Considered when the etiology is multifactorial. All the causal factors can act individually or cumulatively to produce the outcome
• Bradford Hill's criteria for making causal inferences


  • Strength of association
  • Dose-response relationship
  • Lack of temporal ambiguity
  • Consistency of findings
  • Biologic plausibility
  • Coherence of evidence
  • Specificity of association
• Strength of association


  Measured by the relative risk (or odds ratio). The stronger the association, the more it is that the relation is causal.
• Strength of association


  • Risk for development of lung cancer is 8.6 times higher in smokers than in non-smokers
• Dose-response relationship


  As the dose of exposure increases, the risk of disease also increases. If present, it is strong evidence for a causal relationship.
• Dose-response relationship


  • Daily average cigarettes smoked
  • Relative risk of developing lung cancer: 1-14 (6.7), 15-24 (12.3), 25+ (23.7)
• Lack of temporal ambiguity


  Exposure to the factor must have occurred before the disease developed. The temporal relationship is important in regard to the length of the interval between exposure and disease.
• Lack of temporal ambiguity


  • Consumption of contaminated food should precede the symptoms of food poisoning
• Consistency of findings


  The relationship should be found consistently in different studies and in different populations. Unless there is a clear reason to expect different results, replication of the findings should be there.
• Biologic plausibility


  Biologic plausibility refers to coherence with the current body of biologic knowledge. Epidemiologic findings should be consistent with existing biologic knowledge.
• Biologic plausibility


  • Carcinogens from cigarette smoke deposits in the lung over a period of time leading to lung cancer
• Coherence of evidence


  If a relationship is causal, we would expect the findings to be consistent with other data. For the appraisal of causal significance of an association it should be coherent with known facts that are thought to be relevant.
• Coherence of evidence


  • Prevalence of H.pylori is same in men as in women. Incidence of duodenal ulcer in both have been proved to be equal in recent years. Prevalence of peptic ulcer disease is believed to have peaked in the latter part of 19th century cause of poor living standards.
• Specificity of association


  Association is specific when a certain exposure is associated with only one disease. When specificity of an association is found, it provides additional support for a causal inference.
• Specificity of association


  • Prevalence of H.pylori in patients with duodenal ulcer is 90% to 100%, but it is found even in some patients of gastric ulcer and even in asymptomatic individuals.
• Few other criteria which might be useful are: Cessation of exposure - Risk of the disease declines when exposure to the factor is reduced or eliminated, and Consideration of alternate explanations - Extent to which the investigators have taken other possible explanations into account and the extent to which they have ruled out such explanations are important considerations.