Absence of evidence, and evidence of absence

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Reply Tue 20 Oct, 2009 06:58 pm
Former Secretary of Defense, Donald Rumsfeld once said that the absence of evidence is not evidence of absence. I think that is true sometimes, but not true other times. It depends on the circumstances. (And I think this issue is especially interesting with respect to the existence of God). What do you think?
 
prothero
 
Reply Tue 20 Oct, 2009 07:40 pm
@kennethamy,
Well let us try say
the possiblity of nuclear, biological or chemical weapons stored in Iraq but still not found
versus say
A pink unicorn somewhere in the unexplored jungle?
 
kennethamy
 
Reply Tue 20 Oct, 2009 08:27 pm
@prothero,
prothero;98893 wrote:
Well let us try say
the possiblity of nuclear, biological or chemical weapons stored in Iraq but still not found
versus say
A pink unicorn somewhere in the unexplored jungle?


I don't understand what point you are trying to make.
 
Aedes
 
Reply Tue 20 Oct, 2009 08:38 pm
@kennethamy,
What he's saying, Kenneth, is in the case of Iraq the plausibility of there truly being WMDs decreased with the pervasive lack of evidence. But with a pink unicorn in the unexplored jungle, the plausibility is zilch to begin with. So in the former case, absence of evidence (despite a rather thorough search) was evidence of absence, whereas in the latter case absence of evidence is more like no **** sherlock.
 
kennethamy
 
Reply Tue 20 Oct, 2009 08:57 pm
@Aedes,
Aedes;98897 wrote:
What he's saying, Kenneth, is in the case of Iraq the plausibility of there truly being WMDs decreased with the pervasive lack of evidence. But with a pink unicorn in the unexplored jungle, the plausibility is zilch to begin with. So in the former case, absence of evidence (despite a rather thorough search) was evidence of absence, whereas in the latter case absence of evidence is more like no **** sherlock.


But, in the case of Iraq, the issue was whether there were ever any WMDs. Not whether there were still WMDs. Obviously there were no WMDs when we invaded. But that may have been because the Iraqis got rid of them some how. So, was the absence of evidence of WMDs evidence that there had been no WMDs? So what Rumsfeld meant was that absence of evidence after we invaded, was not evidence of the absence of WMDs before we invaded. In the case of the unicorn, if there is no evidence of a unicorn, is that evidence that there are no unicorns? Why? (We know, of course, that the lack of evidence is explained by the fact that there are no unicorns. But that is because we already know there are no unicorns. Why does the absence of evidence show there are no unicorns?) This is trickier than it seems to be.
 
Aedes
 
Reply Tue 20 Oct, 2009 09:11 pm
@kennethamy,
kennethamy;98902 wrote:
But, in the case of Iraq, the issue was whether there were ever any WMDs.
That wasn't an issue because he was well known to have used them before, most famously against Iran but also against the Kurds.

kennethamy;98902 wrote:
Not whether there were still WMDs.
That was the only issue, because the invasion was in large part predicated on his failure to comply with inspectors and the perceived risk of his regime having such weapons.

kennethamy;98902 wrote:
Obviously there were no WMDs when we invaded.
As we ultimately concluded based on the absence of evidence. But in Colin Powell's speech to the UN prior to the invasion he certainly brought up a lot of affirmative evidence (that proved to be wrong).

kennethamy;98902 wrote:
But that may have been because the Iraqis got rid of them some how. So, was the absence of evidence of WMDs evidence that there had been no WMDs? So what Rumsfeld meant was that absence of evidence after we invaded, was not evidence of the absence of WMDs before we invaded.
Ok, I see your point here. Problem is, we will never know -- and when we go back to judge whether or not that war was wise, all we'll have is a circumstantial case for WMDs that was never corroborated on the ground.

kennethamy;98902 wrote:
In the case of the unicorn, if there is no evidence of a unicorn, is that evidence that there are no unicorns?
Of course it's evidence that there are none, assuming we strictly define and agree upon what this putative unicorn is. Looking under your bed for a unicorn is weak evidence that they don't exist at all, but because it corroborates the null hypothesis (i.e. "unicorns do not exist) it is evidence.

Speaking from conventional wisdom, the burden of proof for something that's highly implausible is on affirmative demonstration anyway, right? I mean I'll believe in 5-legged Klingon-speaking siamese wookies the day you show me one.
 
kennethamy
 
Reply Tue 20 Oct, 2009 09:27 pm
@Aedes,
Aedes;98907 wrote:

Ok, I see your point here. Problem is, we will never know -- and when we go back to judge whether or not that war was wise, all we'll have is a circumstantial case for WMDs that was never corroborated on the ground.

Of course it's evidence that there are none, assuming we strictly define and agree upon what this putative unicorn is. Looking under your bed for a unicorn is weak evidence that they don't exist at all, but because it corroborates the null hypothesis (i.e. "unicorns do not exist) it is evidence.

Speaking from conventional wisdom, the burden of proof for something that's highly implausible is on affirmative demonstration anyway, right? I mean I'll believe in 5-legged Klingon-speaking siamese wookies the day you show me one.


It is not a question of never knowing. It is a question of whether the fact that we did not find evidence of WMDs after we invaded was evidence that there were no WMDs before we invaded, and which, would have, in part, justified our invasion. What Rumsfeld is saying is that the fact that we did not find WMDs after we invaded is no reason to think that our invasion was not justified, since there might have been WMDs which were gotten rid of when we invaded.

In the unicorn case, of course finding no evidence of unicorns is evidence there are no unicorns, just as if we were in India, finding no evidence of elephants would be evidence that there were no elephants. The question is whether finding no evidence in both cases is evidence that there are no unicorns (in the one case) and no elephants in the other case. Obviously just finding no evidence of elephants is not enough evidence that there are no elephants; so why should not finding evidence of unicorns be evidence that there are not unicorns? What is difference? Is the fact that there is no evidence of Santa Claus (sufficient) evidence that there is no Santa?
 
Aedes
 
Reply Tue 20 Oct, 2009 09:47 pm
@kennethamy,
kennethamy;98911 wrote:
It is not a question of never knowing. It is a question of whether the fact that we did not find evidence of WMDs after we invaded was evidence that there were no WMDs before we invaded... What Rumsfeld is saying is that the fact that we did not find WMDs after we invaded is no reason to think that our invasion was not justified, since there might have been WMDs which were gotten rid of when we invaded.
This isn't an epistemological exercise, though. This is about building a case. I can accept that absence of evidence just is what it is, I mean I operate under the same mentality in medicine and science.

But the pre-invasion evidence was crappy and based in large measure on that crazy Iraqi informant "Curveball"; very prominent inspectors like Joseph Wilson and Scott Ritter had argued pre-invasion that the WMD programs had been disbanded years before and that this whole uranium entreaty to Niger was bogus -- and when you combine this with the absence of evidence within Iraq, you build a good case that they were not there in recent years before the invasion either. In fact this case outweighs the contrary case.

kennethamy;98911 wrote:
In the unicorn case, of course finding no evidence of unicorns is evidence there are no unicorns, just as if we were in India, finding no evidence of elephants would be evidence that there were no elephants. The question is whether finding no evidence in both cases is evidence that there are no unicorns (in the one case) and no elephants in the other case....What is difference?
The difference is that in the case of elephants their existence is corroborated by other evidence, whereas this is not the case with unicorns.

kennethamy;98911 wrote:
Is the fact that there is no evidence of Santa Claus (sufficient) evidence that there is no Santa?
Asking whether it's evidence is different than whether it is sufficient evidence. I've been making the case that absence of evidence for WMDs IS INDEED evidence that they did not exist. But it is not sufficient in that it doesn't disprove it beyond all doubt.

Think of this in terms of diagnostic tests and their statistical basis. The most useful idea here isnegative predictive value (NPV), which is:

(true negatives) / (true negatives + false negatives)

In other words, a test with a NPV of 100% means that every negative test is a true negative. A test with a NPV of 90% means that only 9 of every 10 negative tests is a true negative.

Looking for WMDs on the ground in Iraq is not a perfect test -- the NPV cannot be 100% ever unless you search every possible square inch above and below ground of the entire country, in the walls, in the caves, everywhere -- and you exclude the possibilities of exportation or destruction. In other words, there are a lot of reasons why this test might be negative.

But then you need to ask yourself is this likely to be a true negative or not. Which is more plausible -- that Saddam had the wherewithal and resources to quickly destroy a whole WMD program, its weapons, and its resources beyond any trace? That he would rather destroy such weapons than using them against an invading army? Or that the evidence suggesting that they existed had a low positive predictive value and it was a false positive??
 
Pangloss
 
Reply Tue 20 Oct, 2009 09:49 pm
@kennethamy,
We do know that Saddam had WMDs prior to the invasion, because we (the US government) supplied him with them. It's as simple as that. Supposedly they were destroyed more recently, also prior to the invasion, and from all accounts so far, this seems likely.
 
Aedes
 
Reply Tue 20 Oct, 2009 10:00 pm
@Pangloss,
Pangloss;98916 wrote:
We do know that Saddam had WMDs prior to the invasion, because we (the US government) supplied him with them. It's as simple as that.
Really? We gave him the chemical weapons that he launched into Iranian schoolyards, despite the fact that the US Military has not used chemical weapons in combat since 1917 and is signatory to treaties banning them? If this is true, how did you get this Top Secret information?
 
kennethamy
 
Reply Tue 20 Oct, 2009 10:10 pm
@Aedes,
Aedes;98915 wrote:

Asking whether it's evidence is different than whether it is sufficient evidence. I've been making the case that absence of evidence for WMDs IS INDEED evidence that they did not exist. But it is not sufficient in that it doesn't disprove it beyond all doubt.

existed had a low positive predictive value and it was a false positive??


But no one expects sufficient evidence to be evidence that proves it "beyond all doubt". We don't even expect that in a capital case of murder. What we expect is "beyond all reasonable doubt". If the standard were "beyond all doubt" we could never prove anything. And, in the case of the WMDs what "sufficient evidence" meant for evidence which justified a belief (along with other beliefs) that we ought to invade Iraq. As you said, we are not doing (theoretic) epistemology here. My view (to lay my cards on the table) is that the absence of evidence for X, is evidence of the absence of X, only if we would expect there to be evidence of X if X did exist. So, the question for our case is whether if there had been WMD's in Iraq, should we have expected there to be evidence that there were after we invaded. Apparently Rumsfeld thinks that the answer to that is, no. I don't, myself know the answer to that. I am only putting forward the logic of the issue. But, do you agree that is the essential question about whether absence of evidence is evidence of absence? If we should have expected there to be some evidence of X even if X no longer exists, then absence of evidence is evidence of absence. But if not, then absence of evidence is not evidence of absence.

This issue becomes even more interesting in the case of the existence of God. Let us suppose that there is no evidence for God (arguendo). Is that sufficient evidence that there is no God? Sufficient=beyond reasonable doubt.
 
Kroda2003
 
Reply Tue 20 Oct, 2009 10:11 pm
@Aedes,
I've always thought of it like this: absence of evidence for X is evidence against X's existence if we would expect to find evidence for X were X to exist.
 
kennethamy
 
Reply Tue 20 Oct, 2009 10:14 pm
@Kroda2003,
Kroda2003;98919 wrote:
I've always thought of it like this: absence of evidence for X is evidence against X's existence if we would expect to find evidence for X were X to exist.


Yes. See post 11.
 
Pangloss
 
Reply Tue 20 Oct, 2009 10:16 pm
@Aedes,
Aedes;98917 wrote:
Really? We gave him the chemical weapons that he launched into Iranian schoolyards, despite the fact that the US Military has not used chemical weapons in combat since 1917 and is signatory to treaties banning them? If this is true, how did you get this Top Secret information?


It's not "top secret information". I guess you haven't heard of the "Reigle Report", which was presented to a senate committee in 1994. Its summary states:
[quote]
3. The United States provided the Government of Iraq with "dual use" licensed materials which assisted in the development of Iraqi chemical, biological, and missile- system programs, including:(6) chemical warfare agent precursors; chemical warfare agent production facility plans and technical drawings (provided as pesticide production facility plans); chemical warhead filling equipment; biological warfare related materials; missile fabrication equipment; and, missile-system guidance equipment.
4. The United States military planned for the use of chemical and biological weapons
by Iraq by
: discussing the chemical/biological threat in pre-war threat assessments; designating chemical/biological production facilities priority bombing targets; assigning a very high priority to SCUD missile units; and, conferring with the U.S. national laboratories about the hazards associated with the bombings of the chemical, biological, nuclear weapons facilities.[/quote]


 
Kroda2003
 
Reply Tue 20 Oct, 2009 10:25 pm
@kennethamy,
kennethamy;98920 wrote:
Yes. See post 11.


Missed that. Exactly what you said.
 
prothero
 
Reply Wed 21 Oct, 2009 12:37 am
@kennethamy,
The value of evidence is somewhat proportional to the likelihood of the assertion.
ie. Pink Unicorns not very likely, evidence sorely needed.
WMD in Iraq, quite possible, not too much evidence needed to believe. (still wrong though).
 
kennethamy
 
Reply Wed 21 Oct, 2009 06:01 am
@prothero,
prothero;98934 wrote:
The value of evidence is somewhat proportional to the likelihood of the assertion.
ie. Pink Unicorns not very likely, evidence sorely needed.
WMD in Iraq, quite possible, not too much evidence needed to believe. (still wrong though).


But we would not know that the likelihood of unicorns was low if not for our evidence. Likelihood is a function of evidence. But our evidence is not merely that unicorns have never been found; it is that unicorns are (biologically) impossible. There is no place in the theory of evolution for such creatures.
 
Zetherin
 
Reply Wed 21 Oct, 2009 06:56 am
@kennethamy,
Kroda2003 wrote:

I've always thought of it like this: absence of evidence for X is evidence against X's existence if we would expect to find evidence for X were X to exist.


Does anyone know if any analytical philosophers or logicians said this outright? Is this part of some sort of proof or principle related to a school of thought, such logical posivitism?
 
kennethamy
 
Reply Wed 21 Oct, 2009 07:04 am
@Zetherin,
Zetherin;98971 wrote:
Does anyone know if any analytical philosophers or logicians said this outright? Is this part of some sort of proof or principle related to a school of thought, such logical posivitism?


I know that Donald Rumsfeld said it, but I don't think he is an analytical philosopher. I think it is an interesting remark worthy of philosophical analysis. I don't think it is true as it stands. But what I think is true is that the absence of evidence is sometimes evidence of absence, and sometimes not. And why that is true is epistemically interesting too. It clearly has connections with the issue of burden of proof, as well.
 
Zetherin
 
Reply Wed 21 Oct, 2009 07:23 am
@kennethamy,
Well, I can't help but agree with:

kennethamy wrote:

But what I think is true is that the absence of evidence is sometimes evidence of absence, and sometimes not.


I think that is true too. It seems as though one would have to go case by case.

I think your unicorn example proves true - the absence of evidence is evidence of absence. But there are many creatures with which we have had an absence of evidence, but it wasn't evidence of absence. X seems as though it would have to be really possible, and not just logically possible (a unicorn is logically possible, but not really possible). For instance, we discover a few new species of fish every year. We wouldn't say our absence of evidence for these new species is evidence for them not existing, or would we?

But say I conjure the idea of a Hampherfish right now. I describe it's qualities and I even make it fit specie(lly) into a scientific model. I design it so that it has place in the theory of evolution. There currently is absence of evidence. Beyond all reasonable doubt, is the absence of evidence, evidence of absence, in this case? Does the fact that I designed the fish to be really possible and not just logically possible make a difference?
 
 

 
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