# Is absence of evidence evidence of absence? **URL:** \ **Category:** Great Debates\ **Created:** [February 20, 2010, 1:18am UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720 "2010-02-20T01:18:27Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Frank\_apisa](https://avatars.discourse-cdn.com/v4/letter/f/47e85d/32.png) [@Frank\_apisa](https://boards.straightdope.com/u/Frank_apisa)\ **Post date:** [February 20, 2010, 5:56pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/41 "2010-02-20T17:56:43Z")
> [@Superfluous\_Parentheses](#): > > x = “these people own a cat”. I thought that was obvious. As a matter of fact…as your conclusion is stated, that is not correct. The supposed “x” in your argument is “the probability that the people own a cat”…which is quite different. If “x” absolutely is “do the people own a cat”…all that stuff not found does not constitute evidence that they do not own a cat. They may have just bought a cat…or decided to baffle any investigators into whether or not they own a cat. That is why you switched your supposed “x” to “the probability they own a cat” when you talked about your conclusions. Once again…go read the article in Wikipedia. It may help. ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [February 20, 2010, 5:57pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/42 "2010-02-20T17:57:02Z")
> [@Anduril](#): > > In entering a regular-sized room, you found a lack of evidence of the existence of an elephant in it. Question: Is this evidence that an elephant is absent in the room? I think so (and I would imagine, so would most of you). Now what if it’s a mouse we’re talking about, would it change your answer. I don’t think so. Great extension the the example. You can conduct an experiment by putting some food out and watching, or searching for evidence of droppings and not finding them etc. But when you just walk into the room, not seeing a mouse in the middle of it is not evidence there are no meeses around. ---
**Author:** ![Strinka](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/strinka/32/512_2.png) [@Strinka](https://boards.straightdope.com/u/Strinka)\ **Post date:** [February 20, 2010, 6:02pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/43 "2010-02-20T18:02:28Z")
> [@Frank\_apisa](#): > > Arguing “there are no elephants in this room” is not arguing to the question of "is absence of evidence evidence of absence.” > > There is plenty of evidence there is no elephant in the room I am in. To suggest that evidence there is no elephant in this room is “an absence of evidence” just doesn’t work. > > I hope that can be seen without further explanation. > > Evidence that something is not there is NOT an absence of evidence! I think you misunderstand what the assertion “Absence of evidence is not evidence of absence” means. It means that lack of evidence for X is not evidence against X. Clearly, this is false, and you seem to agree. Lack of evidence for X can be evidence against X. Seeing an elephant is evidence for X. Not seeing an elephant is lack of evidence for X. In this case, that is also evidence against X. ---
**Author:** ![Frank\_apisa](https://avatars.discourse-cdn.com/v4/letter/f/47e85d/32.png) [@Frank\_apisa](https://boards.straightdope.com/u/Frank_apisa)\ **Post date:** [February 20, 2010, 6:03pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/44 "2010-02-20T18:03:40Z")
> [@Voyager](#): > > Great extension the the example. You can conduct an experiment by putting some food out and watching, or searching for evidence of droppings and not finding them etc. But when you just walk into the room, not seeing a mouse in the middle of it is not evidence there are no meeses around. “Absence of evidence” and “evidence of absence” are completely different things. Once you acknowledge that…the whole argument comes together. ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [February 20, 2010, 6:03pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/45 "2010-02-20T18:03:54Z")
> [@Frank\_apisa](#): > > Anduril > > The proposition being tested is: Is the lack of evidence for “x”…evidence that “x” does not exist. > > In the elephants in the room situtation…“x” is “there are no elephants in the room.” > > All you have to do is look in the room…and if there are no elephants…THAT IS EVIDENCE THAT THERE ARE NO ELEPHANTS IN THE ROOM. > > "X’ is “there are no elephants.” > > You have plenty of evidence that “x” exists…namely, that “there are no elephants in the room” exists. > > There is no lack of evidence for “x” in that propositon…so it does not play a part in the question. Given this, I wonder if you believe anything is lack of evidence. I don’t know how you can see no elephant. The evidence that is lacking here is the reflection of light from a supposed elephant into your eyes. That would be the evidence of an elephant, and its lack suggests there is none. Ditto for light from elephant poop, smell from elephant poop, and sound waves created by the elephant. In fact, we can list the evidence we’d expect to find if there were an elephant in the room, and the lack of this evidence is evidence there is none. Otherwise, our brains would be overwhelmed by the evidence of no elephants, no hippos, no giraffes and no lions in our room, which seems to be crowded with non-animals! ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [February 20, 2010, 6:05pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/46 "2010-02-20T18:05:55Z")
> [@Frank\_apisa](#): > > I will gladly and freely acknowledge that the lack of evidence that there are no microscopic elephants in the room is not evidence that there are no microscopic elephants in the room. > > Thank you for the help in making my point. That I agree with, since you do not expect to see microscopic elephants. Which is my point. You do expect to see large elements. That is why these examples are different. ---
**Author:** ![Frank\_apisa](https://avatars.discourse-cdn.com/v4/letter/f/47e85d/32.png) [@Frank\_apisa](https://boards.straightdope.com/u/Frank_apisa)\ **Post date:** [February 20, 2010, 6:07pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/47 "2010-02-20T18:07:37Z")
> [@Strinka](#): > > I think you misunderstand what the assertion “Absence of evidence is not evidence of absence” means. > > It means that lack of evidence for X is not evidence against X. > > Clearly, this is false, and you seem to agree. Lack of evidence for X can be evidence against X. > > Seeing an elephant is evidence for X. Not seeing an elephant is lack of evidence for X. In this case, that is also evidence against X. There is something in what you say here, Strinka…and I acknowledge that. However, we are discussing the implications of “a lack of evidence.” There is no “lack of evidence” in “evidence of lack.” They are two different things. And without a hidden agenda…almost everybody in this thread would be arguing with me…rather than against me. Simply because there is no evidence of “x”…is not evidence that “x” does not exist. If you make “x” be something that can be investigated (something where you can search for and find evidence)…it is possible TO FIND EVIDENCE that “x” is lacking. In a finite setting like a room…with “x” being an elephant…you CAN FIND EVIDENCE that there is no elephant in the room. That is NOT a lack of evidence…it is evidence of a lack. ---
**Author:** ![Superfluous\_Parentheses](https://avatars.discourse-cdn.com/v4/letter/s/8edcca/32.png) [@Superfluous\_Parentheses](https://boards.straightdope.com/u/Superfluous_Parentheses)\ **Post date:** [February 20, 2010, 6:08pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/48 "2010-02-20T18:08:08Z")
> [@Frank\_apisa](#): > > As a matter of fact…as your conclusion is stated, that is not correct. > > The supposed “x” in your argument is “the probability that the people own a cat”…which is quite different. > > If “x” absolutely is “do the people own a cat”…all that stuff not found does not constitute evidence that they do not own a cat. They may have just bought a cat…or decided to baffle any investigators into whether or not they own a cat. > > That is why you switched your supposed “x” to “the probability they own a cat” when you talked about your conclusions. > > Once again…go read the article in Wikipedia. It may help. I’m impervious to your condescension, but please don’t keep it up. As you very well know, we are talking about probabilities in this thread. Even if you **did** see an elephant in your room that would not be conclusive 100% proof that there is one. > [@](#): > > We are not working with a lack of evidence here. We have evidence…it just happens to be evidence that there are no elephants in the room. What evidence? All you’ve done is NOT seen an elephant. ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [February 20, 2010, 6:09pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/49 "2010-02-20T18:09:50Z")
> [@Frank\_apisa](#): > > “Absence of evidence” and “evidence of absence” are completely different things. > > Once you acknowledge that…the whole argument comes together. The issue is whether we need to qualify “evidence” in absence of evidence, something everyone except you seems to understand. ---
**Author:** ![SteveG1](https://avatars.discourse-cdn.com/v4/letter/s/bb73d2/32.png) [@SteveG1](https://boards.straightdope.com/u/SteveG1)\ **Post date:** [February 20, 2010, 6:10pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/50 "2010-02-20T18:10:06Z")
> [@Voyager](#): > > This issue has come up in a million threads now. We can try to keep religion out of it if it makes things simpler, but I bet it will get there. > > My view: absence of unexpected evidence is not evidence of absence. Absence of expected evidence is. > > For the alien example: since we do not expect that aliens far away from us in general will be doing anything we are likely to sense, absence of this evidence is not evidence of their absence. It is clearly not evidence for them either. At best, it is evidence that they are not beaming radio waves at us. > > On the other hand, if your model of an alien is one who is buzzing us in UFOs, abducting random people, and crashing more often than the Sheriff in Dukes of Hazzard, the absence of any evidence of their presences over the past 50 years is evidence of their absence. If they existed, we should have seen a sign. > > Example number two. You are an insurance investigator, and one of your policy holders says that a dog he had in the yard for a week during vacation was stolen or escaped. You see food for the dog, and water, and the client claims that a lot of it was eaten. However, investigating the yard yields not one bit of dog poop. What does the absence of that evidence tell you? He had a constipated dog? But seriously, the best way to handle some things is to chalk it up to “we don’t know”. Or we can say “it probably doesn’t exist”. Just because something wasn’t found yet, doesn’t make it nonexistent. That doesn’t mean we have to believe every crackpot idea that comes along. It also means we don’t have to reject every idea. Theory is a good word… We’ll believe this thing which seems to work this way, until it is either disproved or a better explanation presents itself. Skepticism is a good word too… We’ll reject this thing until we see something that suggests it may be possible. ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [February 20, 2010, 6:12pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/51 "2010-02-20T18:12:04Z")
> [@Contrapuntal](#): > > They live in a [doodie free zone?](http://www.doodiefreezone.com/) That’s hilarious. My daughter used to work for a pet sitting service - I think I’ll suggest she start one when she gets done with college. ---
**Author:** ![Superfluous\_Parentheses](https://avatars.discourse-cdn.com/v4/letter/s/8edcca/32.png) [@Superfluous\_Parentheses](https://boards.straightdope.com/u/Superfluous_Parentheses)\ **Post date:** [February 20, 2010, 6:13pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/52 "2010-02-20T18:13:08Z")
> [@Frank\_apisa](#): > > And without a hidden agenda…almost everybody in this thread would be arguing with me…rather than against me. I highly doubt it. Besides, it’s you who has the (not so hidden) agenda. If it makes you feel any better, I admit that it’s possible that there is (or was) an appropriately defined deist creator that started the universe. Seems unlikely to me, but hey, absence of evidence for that kind of god would not indicate anything, since that kind of god wouldn’t leave any evidence anyway. None of that applies to your elephant. ---
**Author:** ![gonzomax](https://avatars.discourse-cdn.com/v4/letter/g/e8c25b/32.png) [@gonzomax](https://boards.straightdope.com/u/gonzomax)\ **Post date:** [February 20, 2010, 6:15pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/53 "2010-02-20T18:15:03Z")
Absence of evidence, where much should be there, is compelling. Absence of evidence from things like UFOs has been held to be proof they do not exist. ---
**Author:** ![Frank\_apisa](https://avatars.discourse-cdn.com/v4/letter/f/47e85d/32.png) [@Frank\_apisa](https://boards.straightdope.com/u/Frank_apisa)\ **Post date:** [February 20, 2010, 6:16pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/54 "2010-02-20T18:16:24Z")
> [@Voyager](#): > > Given this, I wonder if you believe anything is lack of evidence. I don’t know how you can see no elephant. The evidence that is lacking here is the reflection of light from a supposed elephant into your eyes. That would be the evidence of an elephant, and its lack suggests there is none. Ditto for light from elephant poop, smell from elephant poop, and sound waves created by the elephant. In fact, we can list the evidence we’d expect to find if there were an elephant in the room, and the lack of this evidence is evidence there is none. > > Otherwise, our brains would be overwhelmed by the evidence of no elephants, no hippos, no giraffes and no lions in our room, which seems to be crowded with non-animals! I am going to leave this discussion with the end of this post. It is obvious you and the rest are not willing to concede the obvious…not willing to concede what any student or teacher of logic would assert with no equivocation whatsoever. All we will end up doing is “yes it is” “no it isn’t.” My guess is that every article on this subject (with the exception of some, thankfully very few, articles by atheists arguing purely from the perspective of atheism) will agree with my position and none will agree with yours. My guess is every logician…every philosopher…will agree with my position and none with yours. Too bad all that. You people seem otherwise to be so reasonable and logical…I really have trouble understanding your reluctance. But in the interests of not wasting time (and not antagonizing any forum monitors), I am done arguing in this thread. ---
**Author:** ![Strinka](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/strinka/32/512_2.png) [@Strinka](https://boards.straightdope.com/u/Strinka)\ **Post date:** [February 20, 2010, 6:21pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/55 "2010-02-20T18:21:09Z")
Frank, I might suggest reading about [Bayes’ Theorem](http://en.wikipedia.org/wiki/Bayes'_theorem). ---
**Author:** ![SteveG1](https://avatars.discourse-cdn.com/v4/letter/s/bb73d2/32.png) [@SteveG1](https://boards.straightdope.com/u/SteveG1)\ **Post date:** [February 20, 2010, 6:22pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/56 "2010-02-20T18:22:03Z")
> [@Frank\_apisa](#): > > I am going to leave this discussion with the end of this post. > > It is obvious you and the rest are not willing to concede the obvious…not willing to concede what any student or teacher of logic would assert with no equivocation whatsoever. > > All we will end up doing is “yes it is” “no it isn’t.” > > My guess is that every article on this subject (with the exception of some, thankfully very few, articles by atheists arguing purely from the perspective of atheism) will agree with my position and none will agree with yours. > > My guess is every logician…every philosopher…will agree with my position and none with yours. > > Too bad all that. > > You people seem otherwise to be so reasonable and logical…I really have trouble understanding your reluctance. > > But in the interests of not wasting time (and not antagonizing any forum monitors), I am done arguing in this thread. Enlighten us, just what is so obvious, that it pains you to stick around? ---
**Author:** ![twhitt](https://avatars.discourse-cdn.com/v4/letter/t/87869e/32.png) [@twhitt](https://boards.straightdope.com/u/twhitt)\ **Post date:** [February 20, 2010, 6:22pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/57 "2010-02-20T18:22:14Z")
This is _blowing my mind_, man. We have a null hypothesis, which is that there are no elephants in the room. I’m defining elephant to be a standard adult african elephant of typical size and weight. We look in the room, and in so doing we _gather evidence_. We then determine whether or not this evidence is sufficient to reject our hypothesis. In this case, we’re looking for evidence that there is an elephant. Some examples of such evidence may be seeing the elephant, or hearing or smelling it, or seeing its food or droppings. Other kinds of evidence may exist. If we find this evidence, we determine whether or not it’s probable, given that our hypothesis is that there are no elephants in the room. Assuming we’ve collected some evidence and determined that it’s improbable given our hypothesis, congratulations! We’ve rejected the hypothesis. If we _fail_ to find any such evidence that there is an elephant in the room, then we cannot reject our null hypothesis, and our next step might be to design another experiment that would cause the hypothesis to be rejected. We may weigh the room, for example. Who knows. We could also construct the experiment with another null hypothesis; this time, we hypothesize that there is an elephant in the room. We would be looking this time for positive evidence that there’s no elephant in the room in order to disprove our null hypothesis. We look around, and if we are not able to find an elephant, nor are we able to find food or water, then once again we _have collected some evidence_. Our evidence is that there is no elephant food, or that we cannot hear an elephant, or whatever it is that we’ve discovered and recorded. We consider how likely this would be in the event that our null hypothesis were true and there were an elephant in the room. Absence of evidence means something very particular. It means you didn’t look, essentially, or that for whatever reason you cannot look in all places. If you have properly defined your experiment, and properly gathered data, then you don’t have an “absence of evidence.” In the elephant example, you could only reasonably have an absence of evidence if you do not go into or otherwise inspect the room. If you did inspect the room, then you’ve gathered evidence and that’s all there is to it. Absence of evidence is not evidence of absence, it’s just being a poor scientist. ---
**Author:** ![appleciders](https://avatars.discourse-cdn.com/v4/letter/a/85f322/32.png) [@appleciders](https://boards.straightdope.com/u/appleciders)\ **Post date:** [February 20, 2010, 6:27pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/58 "2010-02-20T18:27:40Z")
Absence of evidence _is_ evidence of absence. It’s not, however, proof of absence. ---
**Author:** ![Superfluous\_Parentheses](https://avatars.discourse-cdn.com/v4/letter/s/8edcca/32.png) [@Superfluous\_Parentheses](https://boards.straightdope.com/u/Superfluous_Parentheses)\ **Post date:** [February 20, 2010, 6:28pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/59 "2010-02-20T18:28:58Z")
> [@twhitt](#): > > Absence of evidence means something very particular. It means you didn’t look, essentially, or that for whatever reason you cannot look in all places. If you have properly defined your experiment, and properly gathered data, then you don’t have an “absence of evidence.” In the elephant example, you could only reasonably have an absence of evidence if you do not go into or otherwise inspect the room. If you did inspect the room, then you’ve gathered evidence and that’s all there is to it. > > Absence of evidence is not evidence of absence, it’s just being a poor scientist. I’m fine with this definition, and in that sense, I would agree that absence of evidence isn’t evidence of absence. **Frank** however, seems to have something _obvious_ in mind that he’s incapable of expressing but does lead him to accuse everybody else of having hidden agendas and being stupid. ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [February 20, 2010, 6:31pm UTC](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720/60 "2010-02-20T18:31:25Z")
> [@appleciders](#): > > Absence of evidence _is_ evidence of absence. It’s not, however, proof of absence. Only if the evidence is expected - not otherwise. And it definitely is not proof of anything. [Previous page](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720.md?page=2) [Next page](https://boards.straightdope.com/t/is-absence-of-evidence-evidence-of-absence/529720.md?page=4)