If anyone tells you that absence of evidence is not evidence of absence, you have my permission to slap them. Of course, my permission will not prevent you from getting slapped back or charged with assault. Regardless, absence of evidence is very often evidence of absence, and sometimes strong evidence.
To make this claim precise, I propose we use the Bayesian definition of evidence:
If an observation, D, would be more likely under a hypothesis, H, than under the alternative hypothesis, then D is evidence in favor of H. Conversely, if D is less likely under H than under the alternative, D is evidence against H.
As an example, suppose H is the hypothesis that unicorns exist. Since people have explored most of the world’s land mass, I’d say there’s a 99% chance we would have found unicorns if they existed.
So if D is the fact that we have not found unicorns, the probability of D is only 1% if unicorns exist, and 100% if they don’t. Therefore, D is evidence that unicorns don’t exist, with a likelihood ratio of 100:1.
Let’s consider a more realistic example. In a recent article, The Economist discusses the hypothesis that social media use is a major cause of recent increases in rates of self-harm and suicide among teenage girls. To test this hypothesis, they propose an experiment:
Because smartphones were adopted at different rates in different countries, the timing of any increases they caused in suicides or self-harm should vary on this basis.
But their experiment came up empty:
[W]e could not find any statistical link between changes over time in the prevalence of either mobile-internet subscriptions or self-reported social-media use in a country, and changes over time in that country’s suicide or self-harm hospitalisation rates, for either boys or girls.
But if social media were the sole or main cause of rising levels of suicide or self-harm—rather than just one part of a complex problem—country-level data would probably show signs of their effect.
Since it did not, this negative result is evidence against the hypothesis. It may not be strong evidence; there are other reasons the experiment might have failed. And in light of other evidence, it is still plausible that social media is harmful to mental health.
Nevertheless, in this example, as in any reasonable experiment, absence of evidence is evidence of absence.
[In this 2015 article, I made a similar claim that we should stop saying correlation does not imply causation.]