Absence of Evidence ...

[GreyGeek]

People quite often state wrongly that "Absence of evidence is NOT evidence of absence." Why do they do that?

An excellent question, which Kim Oyhus answers very well:
[img width=400 height=328]http://kim.oyhus.no/pics/AbsenceOfEvidence.gif[/img]

and offers the following anecdote:

Lastly, an anecdote from Roar Lauritzsen about Absence of Evidence:

"Suppose you are a programmer, and you are looking for bugs in a program. At first you cannot sleep at night because you are convinced that there must be a bug somewhere, you just haven't found it yet. To find the bug, you test the program to see if you find something that doesn't work as you expected. If you found something, it would be evidence that there was a bug. If you test the program a lot, and still find no evidence of a bug, this increases your confidence that there is no bug. In other words, it counts as evidence for the absence of a bug, and you are finally able to sleep better.

After a while, your program is thoroughly tested, and you still find no evidence for a bug. You begin to suspect that there might not be a bug after all. However, if there is no bug, you will have no purpose as a programmer. You feel as if your life depends on the existence of a bug. You are now looking for the Bug that will save you. You believe that there must be a Bug, so you test your program even more thoroughly. When you still cannot find any evidence for a Bug, you start to rationalize: Although I cannot find any Bug, that does not prove that there is no Bug. You are now a true believer in the Bug."

[dibl]

Re: Absence of Evidence ...

This question closely approaches debating the number of angels who can dance on the head of a pin.

However, IMHO Kim O. has asserted or assumed an equivalency between two nouns that are not at all equivalent, and then proceeded to demonstrate the "proof", as built upon that false assertion.

"Evidence" is a tangible noun -- there is something substantial there. "Absence" is an intangible. There is no substance in absence. It is a condition of being that I think mathematicians refer to as the "null set". A void. So it does not seem reasonable nor appropriate, in my little pea brain, to draw comparisons between substance and absence.

Today's two cents' worth.

[wizard10000]

Re: Absence of Evidence ...

Sagan's original quote in context (boldface mine) -

Appeal to ignorance — the claim that whatever has not been proved false must be true, and vice versa (e.g. There is no compelling evidence that UFOs are not visiting the Earth; therefore UFOs exist — and there is intelligent life elsewhere in the universe. Or: There may be seventy kazillion other worlds, but not one is known to have the moral advancement of the Earth, so we're still central to the Universe.) This impatience with ambiguity can be criticized in the phrase: absence of evidence is not evidence of absence.
-- Ch. 12 : The Fine Art of Baloney Detection, p. 221

Lauritzen reaffirms this with -

Although I cannot find any Bug, that does not prove that there is no Bug.

[GreyGeek]

Re: Absence of Evidence ...

.....
However, IMHO Kim O. has asserted or assumed an equivalency between two nouns that are not at all equivalent, and then proceeded to demonstrate the "proof", as built upon that false assertion.....

That's what mathematicians do: they "define" things and then make proofs based on those definitions. In math they are generally called "Axioms", things that are accepted as true without proof, or because they cannot be proven. Thus the definition of a point.

His Bayesian statements are correct.

[dibl]

Re: Absence of Evidence ...

His Bayesian statements are correct.

Yes -- if I were a qualified mathematician, I assume I would recognize the truth of that.

When I was in the business of writing large proposals to the U.S. military, we had an acronym "UBA", for "Unsubstantiated Bold Assertion". In general, they are to be avoided, in situations where it is important to be convincing to a skeptical reader.

[woodsmoke]

Re: Absence of Evidence ...

dlbl

I'm thinking...... 7304?
maybe 7305? +/- 2?
woodsmoke

[dibl]

Re: Absence of Evidence ...

I'm thinking...... 7304?

Bahh -- you have clearly not studied the average square micrometers of the typical angel's footprint. The number is at least 9743.

[GreyGeek]

Re: Absence of Evidence ...

Ah, you guys are clearly misinformed!

An Angel's foot print is a quantum dot, not an atom!
I could write the proof for that but there isn't enough room in the margin ...