The Bradford Hill postulates, also known as Hill's criteria for causation, are a set of nine principles used to help determine whether an observed association between a factor and an outcome is causal. They were originally proposed by Sir Austin Bradford Hill in 1965 and are widely used in epidemiology and other fields.
The Nine Postulates:
- Strength of association: A strong association between the factor and the outcome is more likely to be causal than a weak association.
- Consistency: The association should be observed in multiple studies and in different populations.
- Specificity: The factor should be specifically associated with the outcome, and not with other outcomes.
- Temporality: The factor must precede the outcome in time.
- Biological gradient: The risk of the outcome should increase with increasing exposure to the factor.
- Plausibility: The association should be plausible based on existing biological knowledge.
- Coherence: The association should be consistent with other known facts about the disease and the factor.
- Experiment: If possible, the association should be confirmed by experimental studies.
- Analogy: The association should be similar to other known causal associations.
Practical Insights:
- These postulates are not rigid rules but rather guideposts for assessing causation.
- Not all postulates need to be satisfied for a causal relationship to be considered likely.
- The strength of evidence for a causal association is determined by the weight of evidence from multiple studies and different types of evidence.
Example: The association between smoking and lung cancer has been shown to meet many of the Bradford Hill criteria, including:
- Strength of association: A strong association exists between smoking and lung cancer.
- Consistency: This association has been observed in multiple studies and in different populations.
- Specificity: Smoking is specifically associated with lung cancer and not with other cancers.
- Temporality: Smoking must precede lung cancer in time.
- Biological gradient: The risk of lung cancer increases with increasing levels of smoking.
- Plausibility: There are biological mechanisms that explain how smoking can cause lung cancer.
- Coherence: The association is consistent with other known facts about lung cancer and smoking.
The weight of evidence from multiple studies and across multiple Bradford Hill criteria strongly supports the conclusion that smoking causes lung cancer.
