Absence of evidence is not evidence of absence … Or is it?

Image by stevepb / Pixabay. Public Domain.

You might have heard this clever wordplay about how the fact that there is no evidence for something does not mean that this does not exist. People proclaiming “Absence of evidence is not evidence of absence” want to say that just because humans have not found evidence for something, it is not excluded that this does not exist.

Well, yes, it is not excluded, but evidence can be used to infer the presence or not presence of something. “For instance, if a doctor does not find any malignant cells in a patient, finding nothing is evidence of absence of cancer, even though the doctor has not actually detected anything per se.” – Wikipedia

As this shows us, the evidence is one thing and PROOF is another thing. We are not talking about proof here. Do NOT confuse evidence and proof. The proof is a difficult thing and it’s almost always close to impossible. Proof only really exists in mathematics. Evidence, however, does give reason to SUSPECT absence, to consider absence very, very, very likely. So, what’s all this with evidence of absence?

There is always a small, minuscule possibility that the evidence has not been observed yet, but this doesn’t mean that this possibility can and should be used to hang on to outrageously unlikely beliefs. Just the opposite.

When analysing an idea to decide what evidence is necessary to support it, we should always take into account what evidence should be there and how much of it should be there. If there should be a LOT of evidence for something and we know exactly what the evidence should be, then a lack of this evidence does indeed allow us to dismiss this idea.

The basic flow to establish that absence of evidence IS indeed evidence of absence:

• You establish a hypothesis.
• Your search for the most likely evidence for it does not find anything.
• You reduce the likelihood of it being true.
• Search for less likely evidence. You do not find it.
• Reduce the likelihood of it being true even further … etc.

Evidence of absence: NOT observing something does reduce the probability of it being true.

Basically: You want to buy eggs. You expect to be able to buy them in a supermarket near you. You have great confidence of being able to buy eggs there, so you go to the store, expecting that you’ll find eggs. After careful looking, in all the right places and everywhere you would expect to find eggs, you find no eggs. You now have evidence of absence of eggs in the store. Even if there are maybe eggs in the store, maybe somewhere where customers are not allowed to go, your search has nevertheless provided you with evidence of absence against the hypothesis that there are eggs in the store.

The good and very popular example is also the search for the Higgs boson in CERN. This could not have been proven or disproven in a single experiment, but in a huge number of experiments where conducted and a huge number of analyses of the data in those experiments have been done. Each experiment brought them further in the search. They were able to dismiss data where Higgs boson was not found. The calculations earlier told them that LHC must enable them to find the Higgs boson if it exists. So they knew where to look for it. If they did not find in inside the spectrum of energies covered by the Large Hadron Collider, they would consider this as evidence of absence.

Or as Wikipedia put it simply and eloquently:

• An example of evidence of absence is checking your pockets for spare change and finding nothing, but being confident that the search would have found it if it was there.
• A biopsy shows the absence of malignant cells.
• One very carefully inspects the back seat of one's car and finds no adult-sized kangaroos.
• The train schedule does not say that the train stops here at 3:00pm on a Sunday.

Wikipedia

If you are interested, there’s also a real proof of this in the probability theory. You can read more on these links and resources used:

• Evidence of absence – Wikipedia
• Argument from ignorance – Wikipedia
• Absence of evidence – Rationalwiki
• Absence of Evidence is Evidence of Absence – Kim G. S. Øyhus
• Absence of Evidence Is Evidence of Absence – LessWrong
• Null result – Wikipedia