Absence of evidence is not evidence of absence

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Absence IX by Paulo Fehlauer Attribution-NonCommercial-ShareAlike

Its very hard, or perhaps impossible, to prove that something doesn’t have an effect on something else. When we find no evidence of an association, is it just that we haven’t found it yet? Perhaps it doesn’t occur very frequently and our sample is too small. Perhaps a small effect is hidden amongst the variation caused by other factors. As Altman and Bland (1995) explain

“When we are told that “there is no evidence that A causes B” we should first ask whether absence of evidence means simply that there is no information at all.”

Their short article is well worth reading. You can access it here http://www.bmj.com/content/311/7003/485
Altman DG and Bland JM (1995) Statistics notes: Absence of evidence is not evidence of absence BMJ 311:485

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