(Blog contribution by: Pragyan Paramita Parija)
Guidelines for judging whether an observed association is causal:
1. Temporal relationship
2. Strength of the association
3. Dose – response relationship/ Biological gradient
4. Replication of the findings
5. Biologic plausibility
6. Consideration of alternate explanations
7. Cessation of exposure
8. Consistency with other knowledge
9. Specificity of the association
1. Temporal relationship:
If a factor is believed to be the cause of a disease, exposure to the factor must have occurred before the disease developed. It is easier to establish a temporal relationship in a prospective cohort study than in a retrospective cohort study or case control study. It not only clarifies the order in which the two occur but also in regard to length of the interval between exposure and disease. E.g. Asbestos linked to increased risk for lung carcinoma, but the latent period between exposure and outcome is 15 – 20 years. If a lung cancer occurs 3 years after asbestos exposure, it may not be the cause of lung cancer.
2. Strength of the association:
Measured by relative risk or odds ratio. Stronger the association, more likely the relation is causal.
3. Dose response relationship/Biological gradient:
As the dose of exposure increases, the risk of the disease also increases.
4. Replication of the findings:
If the relationship is causal, we would expect to find it consistently in different studies and in different populations.
5. Biologic Plausibility:
It refers to coherence with the current body of biologic knowledge.
6. Consideration of alternate explanations:
An observed association interpreted in regard to whether a relationship is causal or is the result of confounding. The 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.
7. Cessation of exposure:
If a factor is a cause of a disease, we would expect the risk of the disease to decline when exposure to the factor is reduced or eliminated.
8. Consistency with other knowledge:
If a relationship is causal, we would expect the findings to be consistent with other data.
9. Specificity of the association:
When a certain exposure is associated with only one disease. It is the weakest criteria.
If one cause has been shown to result in a particular effect, the same cause is most likely to result into other effects of similar nature. This criterion may not always work.
E.g. Culex mosquito manifest lymphadenitis, but other causes with lymphadenitis may not be transmitted by mosquito.