Absence of evidence fallacy:
| Definition | Example |
| When it is argued that finding no evidence for something is no evidence for the absence of that thing. | The fact that you did not see me at your birthday party does not mean I was not there! |
| Notes | |
This argument, attributed to Carl Sagan, is often invoked when claims of a god comes under scrutiny. While the absence of evidence is not proof of absence, it is, to varying degrees, evidence of absence. The degree of evidence an absence of evidence is for the absence of anything will depend on the context. If there is no evidence of thing X in a location Y, the degree that will constitute evidence for the absence of that thing depends upon how we substantiate the variables. Is X a molecule, mouse or monster? Is Y a cup, couch or continent? The expected discoverability of X within its relevant logical or spacial space Y must be considered.
—J.P. Moreland and W.L. Craig, Philosophical Foundations for a Christian Worldview |
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Case Study One
After a Harvard Medical School study on the effects of prayer on heart surgery patients showed no statistical advantage of prayer, it was wrongly argued that “the absence of evidence is not evidence of absence”.
Case Study Two
Those who claim they have the biblical Holy Spirit helping them to make predictions or avoid logical fallacies in discussion, when confronted with evidence that they are, at best, only on par with those who do not claim to have a Holy Spirit, they often invoke this absence of evidence fallacy. They then often claim the burden of proof falls on those denying the existence of their Holy Spirit.
Case Study Three
Donald Rumsfeld invoked the “absence of evidence” fallacy in reference to possible weapons of mass destruction in Iraq. Having not found WMDs where they were expected was, in fact, evidence of their absence.
Keep in mind that a fallacious argument does not entail an erroneous position.
Logfall - Logical Fallacies 
For more info on this, google for proof that absence of evidence is evidence of absence using conditional probability.
To summarize what it says, absence of evidence IS evidence of absence, but it is not PROOF of absence or even necessarily STRONG evidence.
Usually, absence of evidence is only WEAK evidence of absence.
Most confusion on this comes from people mixing up the meanings of proof and evidence, as well as how strong the evidence is. So for clarification, here’s the meanings of the terms used in the statement.
Proof:
Offers complete certainty. (think ala math or deductive logic proof) .
Evidence:
Increases certainty, but does not render it complete (think as in probability or inductive arguments).
The above is not sound and actually commits the fallacy. You cannot use an absence of something as evidence in your favor.
Todd, here are a few additional examples in which the absence of evidence is indeed evidence of absence.
1. The man who has been living next door for 10 years claims to have a wife living with him. To the degree (length of time living there and amount of observation) that you have not seen a woman next door, to that degree you are warranted in doubting his claim.
2. A female friend claims to be 8 months pregnant. To the degree she does not exhibit the round belly nomally associated with pregnant women, to this degree you have evidence to doubt the claim.
3. A man claiming to have clairvoyance submits to a test of his claim, but fails to identify the objects printed on the hidden side of cards above what would be expected from chance. The absence of success above chance constitutes evidence that he lacks clairvoyance.
To the degree that there is an absence of effects normally associated with the presence of a given entity or state of existence, to that degree we can consider that lack of confirming evidence disconfirming evidence.
The question of “Does AOE = EOA?” depends upon the reproducibility of AOE. If 1 Person does not see Me at a party, it does not mean Other did not and, yes, the argument would then be “weak”. However, if Nobody at that party saw Me, the likelihood I was not there increases. Applying the “AOE = EOA” argument to the existence of any particularly alleged Deity is quite possibly fallacious in itself, however, because alleged Deities to which the “AOE = EOA” is applied are, in My experience, Those which allegedly have the same degree of choice which Any Other Person has and the tests for which “AOE = EOA” is applied are, in My experience, often those which would require the alleged Deity to actively participate. Since Everyone is often free to choose whether or not to actively participate in tests, testing an alleged Deity which has that same freedom to choose whether or not to actively participate is about as logically sound as saying, “Because Thomas Shmalahazen didn’t show up to participate in this test, Thomas must not exist.” However, if I understand correctly, such a fallacy is a different one than the one listed on this page.
Any volitional and omnipotent god can choose to hide, but the definition of that god must then include the notion that this god chooses to hide. Only then can the apparent absence of that god be consistent with its existence.
But even when a child is playing hide-and-seek with a “friend” whom their parents has not previously seen, there comes a point in their search around the “house” at which your parents are justified in concluding that your “friend” most likely never existed.
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In the example given:
“Donald Rumsfeld invoked the “absence of evidence” fallacy in reference to possible weapons of mass destruction in Iraq. Having not found WMDs where they were expected was, in fact, evidence of their absence.”
The conclusion might look good on the surface, however, it can be viewed differently. For instance the statement of, “Having not found WMDs where they were expected was, in fact, evidence of their absence.” can be be viewed as the WMD’s did not nor ever had existed there, or it can be viewed as them not being there at that moment. However, the arguing that their absence shows they did not exist falls short if they had existed at the time, but were moved elsewhere before the ones searching for them got there. This shows there absence is not proof of their non-existence. So again, the argument from absence does not prove they were not there only that they are not currently there.
‘This is about evidence of absence rather than “proof” of absence. The difference is substantial.
Exactly, but in the statement a the absent was used to say for sure beyond a shadow of doubt there were no WMDs. That is unsubstantiated and therefore violates the argument from absence law.
Are you somehow in disagreement with the notion that absence of evidence is evidence (not proof) of absence?