association and causation


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association and causation

  2. 2. ASSOCIATION <ul><li>If two attributes say A and B are found to co-exit more often than an ordinary chance. </li></ul><ul><li>It is useful to consider the concept of correlation. </li></ul><ul><li>Correlation indicates the degree of association between two variables. </li></ul>
  3. 3. CAUSATION <ul><li>If one of these attributes say A is the suspected cause and the other say B is a disease then we have a reason to suspect that A has caused B. </li></ul>
  4. 4. Types Of Association <ul><li>1. Spurious association </li></ul><ul><li>2. Indirect association </li></ul><ul><li>3. Direct association </li></ul><ul><ul><li>One-to-one causal association </li></ul></ul><ul><ul><li>Multifactorial causation </li></ul></ul>
  5. 5. Spurious Association <ul><li>This is an association which appears due to improper comparison. </li></ul><ul><li>Observed association between a disease and suspected factor may not be real. </li></ul><ul><li>E.g.; Neonatal mortality was observed to be more in the newborns born in a hospital than those born at home. This is likely to lead to a conclusion that home delivery is better for the health of newborn. </li></ul><ul><li>However, this conclusion was not drawn in the study because the proportion of “high risk” deliveries was found to be higher in the hospital than in home. </li></ul>
  6. 6. Indirect Association <ul><li>It is a statistical association between a characteristic of interest and a disease due to the presence of another factor i.e. common factor (confounding variable). </li></ul><ul><li>E.g.: Neonatal mortality (A) was found to be associated with maternal age above 30 years (B) and with birth order 4 and above (c). </li></ul><ul><li>It was also shown that the attribute B and C were associated with each other. </li></ul>
  7. 7. Direct Association <ul><li>The association between the two attributes is not through the third attributes. </li></ul><ul><li>When the study reveals it is not a spurious association. </li></ul><ul><li>When the disease is present, the factor must also be present. </li></ul>
  8. 8. Direct Association Is Classified Into Two Types <ul><li>One-to-one Casual Relationship: </li></ul><ul><li>The variables are stated to be casual related (AB) if a change in A is followed by a change in B. </li></ul><ul><li>When the disease is present, the factor must also be present. </li></ul><ul><li>A single factor or cause may lead to more than one outcome. </li></ul>
  9. 9. Direct Association Is Classified Into Two Types <ul><li>Multifactorial causation: </li></ul><ul><li>Alternative causal factors each acting independently. </li></ul><ul><li>E.g. In lung cancer more than one factor (e.g. air pollution, smoking, heredity) can produce the disease independently. </li></ul>
  10. 10. Six Guidelines For Judging Whether An Association Is Causal <ul><li>Temporal Association </li></ul><ul><li>Consistency Of The Association </li></ul><ul><li>Specificity Of The Association </li></ul><ul><li>Strength Of The Association </li></ul><ul><li>Coherence Of The Association </li></ul><ul><li>Biologic Plausibility Of The Association </li></ul>
  11. 11. Temporal association <ul><li>The causal attribute must precede the disease or unfavorable outcome. </li></ul><ul><li>Exposure to the factor must have occurred before the disease developed. </li></ul><ul><li>Length of interval between exposure and disease very important </li></ul><ul><ul><li>If the disease develops in a period of time too soon after exposure, the causal relationship is called into question. </li></ul></ul>
  12. 12. Consistency Of The Association <ul><li>Consistency is the occurrence of the association at some other time and place repeatedly. </li></ul><ul><li>If a relationship is causal, the findings should be consistent with other data. </li></ul><ul><li>If lung cancer incidence increased as cigarette use was on the decline, we would have to be able to explain how this was consistent with a causal relationship. </li></ul><ul><li>If there is no consistency it will weaken a causal interpretation. </li></ul><ul><li>The causal association between smoking and lung cancer due to its consistency. </li></ul>
  13. 13. Specificity Of The Association <ul><li>The weakest of the criteria (should probably be eliminated) </li></ul><ul><li>Specific exposure is associated with only one disease. </li></ul><ul><li>This is used by tobacco companies to argue that smoking is not causal in lung cancer. </li></ul><ul><ul><li>Smoking is associated with many diseases. </li></ul></ul><ul><li>Specificity implies a one to one relationship between the cause and effect. </li></ul>
  14. 14. Strength Of The Association <ul><li>Relationship between cause and outcome could be strong or weak. </li></ul><ul><li>With increasing level of exposure to the risk factor an increase in incidence of the disease is found. </li></ul><ul><li>There are statistical methods to quantify the strength of association viz; calculation of relative risk, attributable risk etc. </li></ul>
  15. 15. Coherence Of The Association <ul><li>Causal significance of an association is its unity with known facts that are thought to be related. </li></ul><ul><li>E.g.: the rising consumption of tobacco in the form of cigarettes and the rising incidence of lung cancer are coherent. </li></ul>
  16. 16. Biologic Plausibility Of The Association <ul><li>The association must be consistent with the other knowledge (viz mechanism of action, evidence from animal experiments etc). </li></ul><ul><li>Sometimes the lack of plausibility may simply be due to the lack of sufficient knowledge about the pathogenesis of a disease. </li></ul>