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Evidence of absence

The other day, reading Tom Chivers' excellent book on Bayesian statistics Everything is Predictable, I was reminded of that old chestnut, 'absence of evidence is not evidence of absence.' This is often put forward as if it were a powerful logical argument. But, in reality, it's a bit of common sense that sometimes works, but always oversimplifies.

In case you aren't familiar with the expression, I might say that I've never seen any evidence that dark matter exists (as opposed to the behaviour of galaxies and galactic clusters attributed to dark matter), but I shouldn't take that as evidence that dark matter doesn't exist.

As Tom Chivers points out, this is very frequentist thinking. The Bayesian approach would be that every good quality experiment that fails to find dark matter modifies our priors - it can be used to reduce the probability that it exists.

Interestingly, this somewhat trite saying only tends to be wheeled out when responding to a theory we agree with. I can imagine UFO enthusiasts saying 'just because there isn't good evidence, it doesn't mean they aren't out there.' While technically true, the rest of us would probably say that we're inclined to take the lack of good evidence, particularly now everyone carries a camera all the time, as evidence of absence.

Note that this is totally different to saying that lack of evidence definitely means that something doesn't exist - of course that's not true. Yet if you keep looking for something and fail to find repeatedly, it is logical that this should reduce the probability you apply to its existence. And in practice, science often does this. For years, experiments like the famous Michelson-Morley one attempted to find evidence for the existence of the luminiferous aether. No evidence came to light, and over time the likelihood of it existing was reduced, helped by a theory that made it unnecessary.

The reality, then, is absence of evidence, when that evidence is searched for effectively, is evidence of absence. It's just not proof.

Image from Unsplash by Albert Antony

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  1. Anonymous2 June 2024 at 09:42

    Although I agree with you, I wonder about events that are so rare that the chances of seeing repeated events are mercifully nil. Magnetic monopoles.

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