# Causality and Who’s Running Up That Hill?

Yes, back to the eighties. They were my high school and undergrad years – so very memorable! This song – Running Up That Hill – by Kate Bush was her first great hit from that time.

So, why was she running up that hill you ask? Well, it was because she had finally come to realize the importance of establishing the minimal conditions needed to establish a causal relationship between two entities, of course! Somewhat like the story of Archimedes who leapt from his bath yelling “Eureka” in excitement having discovered a law of physics that would later become the building block to fluid mechanics (see Archimedes principle).

In 1965 (no, I was not born yet – but just!), Austin Bradford Hill a British medical statistician proposed minimal conditions needed to establish a causal relationship between two entities. These later became know as the Hill’s Criteria. Very often people get the relationship of association confused with that of causality. See my previous post Rebel Without a Cause for some insight on when an association can be considered as cause and effect.

Today we will talk about the first of the nine Hill criteria: Strength

– The strength of an association is defined as the size of a given association as measured by appropriate statistical tests.  The stronger the association, the more likely it is that the relation between the two entities of interest is cause and effect.  For example, the more highly correlated hypertension is with smoking, the stronger is the relation between the exposure, smoking, to the outcome, hypertension. Though we cannot be sure of the direction of the relationship (this will be achieved when we discuss Temporality) – as hypertension could hypothetically lead subjects to smoke – we can certainly decide that the strength of the association observed supports our argument of causation.

Look at that, we have completed the first criterium all ready! Next we will look at Consistency.

Have a listen to “Strength Of A Women” by Shaggy to recover from today’s fun and…

… I’ll see you in the blogosphere.

Pascal Tyrrell