The Environment and Disease: Association or Causation?

The Environment and Disease: Association or Causation?


Austin Bradford Hill


The Environment and Disease: Association or Causation?

Austin Bradford Hill 1963. (View Paper →)

More often that not we are dependent upon our observation and enumeration of defined events for which we seek antecedents. In other words we see that the event B is associated with the environmental features A…. in what circumstances can we pass from this observed association to a verdict of causation? Upon what basis should we proceed to do so?

The Bradford Hill criteria help researchers determine if there is a causal relationship between an exposure and an outcome.

I heard the briefest of references to the Bradford Hill criteria on a podcast and decided to look into it. It’s one of those concepts that I could instantly see the value in. Evidently they are widely adopted and accepted. Here’s a brief extract from Wikipedia of the considerations…

  1. Strength (or effect size): A small association does not mean that there is not a causal effect, though the larger the association, the more likely that it is causal.
  2. Consistency (or reproducibility): Consistent findings observed by different persons in different places with different samples strengthens the likelihood of an effect.
  3. Specificity: Causation is likely if there is a very specific population at a specific site and disease with no other likely explanation. The more specific an association between a factor and an effect is, the bigger the probability of a causal relationship.
  4. Temporality: The effect has to occur after the cause (and if there is an expected delay between the cause and expected effect, then the effect must occur after that delay).
  5. Biological gradient (dose-response relationship). Greater exposure should generally lead to greater incidence of the effect. However, in some cases, the mere presence of the factor can trigger the effect. In other cases, an inverse proportion is observed: greater exposure leads to lower incidence.
  6. Plausibility: A plausible mechanism between cause and effect is helpful (but Hill noted that knowledge of the mechanism is limited by current knowledge).
  7. Coherence: Coherence between epidemiological and laboratory findings increases the likelihood of an effect. However, Hill noted that "lack of such [laboratory] evidence cannot nullify the epidemiological effect on associations".
  8. Experiment: "Occasionally it is possible to appeal to experimental evidence".
  9. Analogy: The use of analogies or similarities between the observed association and any other associations.

Some authors also consider Reversibility: If the cause is deleted then the effect should disappear as well.