Bradford Hill criteria

Bradford Hill Criteria

The Bradford Hill criteria, also known as Hill's criteria for causation, are a group of nine principles that can be useful in establishing epidemiologic evidence of a causal relationship between a presumed cause and an observed effect. The criteria were first outlined by Sir Austin Bradford Hill in a 1965 lecture at the Royal Society of Medicine in London. Despite being initially developed for the field of epidemiology, these criteria have since been applied in a wide range of disciplines to assess causal associations.

Background

Sir Austin Bradford Hill was a British medical statistician and epidemiologist. His work, alongside Richard Doll, on the connection between cigarette smoking and lung cancer was groundbreaking. In his 1965 lecture, Hill cautioned against rigid statistical criteria for proving causality and proposed a more flexible approach, which became known as the Bradford Hill criteria.

The Criteria

The Bradford Hill criteria consist of the following nine principles:

1. Strength of Association: The stronger the association between the independent variable (e.g., exposure to a risk factor) and the dependent variable (e.g., disease), the more likely the association is causal.
2. Consistency: The association is consistently observed in different studies, populations, and circumstances.
3. Specificity: A specific population exposed to a particular factor shows the effect, whereas those not exposed do not.
4. Temporality: The cause precedes the effect in time.
5. Biological Gradient: Also known as dose-response curve; greater exposure should generally lead to greater incidence of the effect.
6. Plausibility: The observed association can be plausibly explained by existing biological or social models.
7. Coherence: The association does not seriously conflict with the generally known facts of the natural history and biology of the disease.
8. Experiment: Causal evidence can be supported by experimental or semi-experimental data, such as randomized controlled trials.
9. Analogy: The effect of similar factors may be considered.

Application

The Bradford Hill criteria have been applied in various fields to assess causality. While not all criteria need to be met for a causal relationship to be established, the more criteria that are met, the stronger the evidence for causality. It is important to note that these criteria are meant to guide judgment and are not to be used as a checklist for definitively proving causality.

Limitations

One of the main limitations of the Bradford Hill criteria is that they are qualitative rather than quantitative. This means that their application can be somewhat subjective. Additionally, not all criteria are applicable in every situation, and the importance of each criterion may vary depending on the context.

Conclusion

The Bradford Hill criteria remain a fundamental tool in epidemiology and other disciplines for assessing causal relationships. While they do not provide a definitive checklist for proving causality, they offer a valuable framework for evaluating the evidence of causation.