{"id":1083,"date":"2012-11-18T20:51:33","date_gmt":"2012-11-18T20:51:33","guid":{"rendered":"https:\/\/skepticink.com\/tippling\/?p=1083"},"modified":"2012-11-19T20:01:03","modified_gmt":"2012-11-19T20:01:03","slug":"absence-of-evidence-is-evidence-of-absence-in-many-cases","status":"publish","type":"post","link":"https:\/\/skepticink.com\/tippling\/2012\/11\/18\/absence-of-evidence-is-evidence-of-absence-in-many-cases\/","title":{"rendered":"Absence of evidence IS evidence of absence (in many cases)"},"content":{"rendered":"

I get annoyed at apologists and theists that trot “absence of evidence is not evidence of absence” out as if it is some kind of unbreakable law. Nonsense. Not only is it the basis of a gazillion court cases, as well as carefully constructed scientific experiments, it is very useful to boot. It CAN be a fallacy when formulated as an Argument from Ignorance, but it is not strictly synonymous. John D. Cook, mathematician and statistician, sums it up wel<\/a>l. Also, Carrier in Proving History deals really well with it as not being a fallacy in historical analyses. JohnM has recently being claiming this mantra<\/a> incorrectly.<\/p>\n

Absence of evidence<\/h1>\n

by\u00a0JOHN\u00a0on\u00a0FEBRUARY 22, 2011<\/abbr><\/p>\n

\n

Here\u2019s a little saying that irritates me:<\/p>\n

Absence of evidence is not evidence of absence.<\/p><\/blockquote>\n

It\u2019s the kind of thing a Sherlock Holmes-like character might say in a detective novel. The idea is that we can\u2019t be sure something doesn\u2019t exist just because we haven\u2019t seen it yet.<\/p>\n

What bothers me is that the statement misuses the word \u201cevidence.\u201d The statement would be correct if we substituted \u201cproof\u201d for \u201cevidence.\u201d We can\u2019t conclude with absolute certainty that something doesn\u2019t exist just because we haven\u2019t yet proved that it does. But\u00a0evidence<\/em>\u00a0is not the same as\u00a0proof<\/em>.<\/p>\n

Why do we believe that dodo birds are extinct? Because no one has seen one in three centuries. That is, there is an absence of evidence that they exist. That is tantamount to evidence that they do not exist. It\u2019s logically possible that a dodo bird is alive and well somewhere, but there is overwhelming evidence to suggest this is not the case.<\/p>\n

Evidence can lead to the wrong conclusion. Why did scientists believe that the coelacanth was extinct? Because no one had seen one except in fossils. The species was believed to have gone extinct 65 million years ago. But in 1938 a fisherman caught one. Absence of evidence is not\u00a0proof<\/em>\u00a0of absence.<\/p>\n

\"coelacanth,<\/p>\n

Though it is not proof, absence of evidence is\u00a0unusually strong<\/strong>\u00a0<\/em>evidence due to subtle statistical result. Compare the following two scenarios.<\/p>\n

Scenario 1<\/strong>: You\u2019ve sequenced the DNA of a large number prostate tumors and found that not one had a particular genetic mutation. How confident can you be that prostate tumors never have this mutation?<\/p>\n

Scenario 2<\/strong>: You\u2019ve found that 40% of prostate tumors in your sample have a particular mutation. How confident can you be that 40% of all prostate tumors have this mutation?<\/p>\n

It turns out you can have more confidence in the first scenario than the second. If you\u2019ve tested\u00a0N<\/em>\u00a0subjects and not found the mutation, the length of your confidence interval around zero is proportional to\u00a0N<\/em>. But if you\u2019ve tested\u00a0N<\/em>\u00a0subjects and found the mutation in 40% of subjects, the length of your confidence interval around 0.40 is proportional to ?N<\/em>. So, for example, if\u00a0N<\/em>\u00a0= 10,000 then the former interval has length on the order of 1\/10,000 while the latter interval has length on the order of 1\/100. This is known as the\u00a0rule of three<\/strong>. You can find both a frequentist and a Bayesian justification of the rule\u00a0here<\/a>.<\/p>\n

Absence of evidence is unusually strong evidence that something is at least rare, though it\u2019s not proof. Sometimes you catch a coelacanth.<\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

I get annoyed at apologists and theists that trot “absence of evidence is not evidence of absence” out as if it is some kind of unbreakable law. Nonsense. Not only is it the basis of a gazillion court cases, as well as carefully constructed scientific experiments, it is very useful to boot. It CAN be a fallacy when formulated as an Argument from Ignorance, but it is not strictly synonymous. John D. Cook, mathematician and statistician, sums it up well. Also, Carrier in Proving History deals really well with it as not being a fallacy in historical analyses. JohnM has recently being claiming this mantra incorrectly.<\/p>\n","protected":false},"author":8,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[169],"tags":[258,259],"class_list":["post-1083","post","type-post","status-publish","format-standard","hentry","category-epistemology-2","tag-absence-of-evidence","tag-john-d-cook"],"_links":{"self":[{"href":"https:\/\/skepticink.com\/tippling\/wp-json\/wp\/v2\/posts\/1083","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/skepticink.com\/tippling\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/skepticink.com\/tippling\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/skepticink.com\/tippling\/wp-json\/wp\/v2\/users\/8"}],"replies":[{"embeddable":true,"href":"https:\/\/skepticink.com\/tippling\/wp-json\/wp\/v2\/comments?post=1083"}],"version-history":[{"count":0,"href":"https:\/\/skepticink.com\/tippling\/wp-json\/wp\/v2\/posts\/1083\/revisions"}],"wp:attachment":[{"href":"https:\/\/skepticink.com\/tippling\/wp-json\/wp\/v2\/media?parent=1083"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/skepticink.com\/tippling\/wp-json\/wp\/v2\/categories?post=1083"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/skepticink.com\/tippling\/wp-json\/wp\/v2\/tags?post=1083"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}