A look at Causality:

combined

As an Engineer, I am very interested in Cause and Effect phenomenon. We have always been trained to understand that correlation does not mean causality. But then how does one establish causality? The most common method is to use randomized blind studies.

In this post I want to highlight two pioneers who have established paradigms for understanding cause and effect. They are David Fume and Sir Bradford Hill. Understanding their approaches provides a deeper philosophical understanding on cause and effect.

David Hume:

David Hume (1711 – 1776) was a great Scottish Philosopher who authored “A treatise of human nature.” In this, he gives eight rules to clearly determine cause and effect. I have paraphrased some of them for clarity.

  1. The cause and effect must be contiguous in space and time.
  2. The cause must be prior to the effect.
  3. There must be a constant union between the cause and effect. It is chiefly this quality that constitutes the relation.
  4. The same cause always produces the same effect, and the same effect arises from the same cause.
  5. To add to rule 4, where several different objects produce the same effect, it must be by means of some quality, which we discover to be common amongst them.
  6. The difference in the effects of two resembling objects must proceed from that particular, in which they differ. For as like causes always produce like effects, when in any instance we find our expectation to be disappointed, we must conclude that this irregularity proceeds from some difference in the causes.
  7. When any object increases or diminishes with the increase or diminution of its cause, it is to be regarded as a compounded effect, derived from the union of several different effects, which arise from the several different parts of the cause. The absence or presence of one part of the cause is supposed to always be matched with the absence or presence of a proportional part of the effect. This constant conjunction sufficiently proves that the one part is the cause of the other.
  8. An object, which exists for any time in its full perfection without any effect, is not the sole cause of that effect, but requires to be assisted by some other principle, which may forward its influence and operation. For as like effects necessarily follow from like causes, and in a contiguous time and place, their separation for a moment shows, that these causes are not complete ones.

The particular section of Hume’s treatise is available here. https://ebooks.adelaide.edu.au/h/hume/david/h92t/B1.3.15.html

Sir Bradford Hill:

Sir Bradford Hill (1897–1991) is most famous for the Hill criteria in epidemiology. The hill criteria, described first in his paper “The Environment and Disease: Association or Causation”, again provides a list of rules to determine causality. As a side note, his paper used smoking-cancer link as an example, and this was refuted strongly by Sir Ronald Fischer. That will be a post for another day.

The Hill criteria consist of 9 rules, and there are some overlaps with Hume’s rules. Hill has given the following background for his criteria.

“None of these nine viewpoints can bring indisputable evidence for or against a cause and effect hypothesis …. What they can do, with greater or less strength, is to help answer the fundamental question—is there any other way of explaining the set of facts before us, is there any other answer equally, or more, likely than cause and effect?”

  1. Strength: A strong correlation is a pre-requisite to consider causality.
  2. Consistency: To quote Hill “Has it been repeatedly observed by different persons, in different places, circumstances and times?” This answers the reproducibility question. Is the effect reproducible under different set of criteria?
  3. Specificity: Ideally, in epidemiology, an effect is linked with one specific cause. In Hill’s eyes, the more specific the association to a specific cause, the stronger the argument is in favor of causation. In his words, “If the association is limited to specific workers and to particular sites and types of disease and there is no association between the work and other modes of dying, then clearly that is a strong argument in favor of causation.”
  4. Temporality: Hill presented this criterion as a question – which is the cart (effect) and which is the horse (cause)? As noted before, the cause must always come before the effect, on a timeline.
  5. Biological gradient: The keyword here is “gradient”. Is there a comparable increase in effect, with an increase in the cause factor? In his paper, Hill gave the example of death rate to number of cigarettes smoked in a day. This is a kind of feedback loop.
  6. Plausibility: This is my favorite criterion in the list. Is the cause and effect theory likely to happen from a logic stand point? I was very excited when I saw that Hill actually quoted Sherlock Holmes in his paper. “In short, the association we observe may be one new to science or medicine and we must not dismiss it too light-heartedly as just too odd. As Sherlock Holmes advised Dr Watson, ‘when you have eliminated the impossible, whatever remains, however improbable, must be the truth.’”
  7. Coherence: To expand upon criterion 6, Hill wanted to make sure that the cause and effect theory did not go against existing theories. There is always more buy-in when the theory does not conflict with existing knowledge.
  8. Experiment: One must always provide experimental evidence to back up the cause and effect theory.
  9. Analogy: If there is supporting evidence on similar factors, it makes it easier to establish causality.

Bradford Hill’s seminal paper is available here. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1898525/pdf/procrsmed00196-0010.pdf

Final words:

One must learn from great thinkers from other fields. In your next problem solving project, when you are trying to perform root cause analysis, will you be able to use Hume’s rules or Hill’s criteria?

Always keep on learning…

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