On 13-Jul-08 19:53:47, Johannes Huesing wrote: > Frank E Harrell Jr <[EMAIL PROTECTED]> [Sun, Jul 13, 2008 at > 08:07:37PM CEST]: >> (Ted Harding) wrote: >>> On 13-Jul-08 13:29:13, Frank E Harrell Jr wrote: >>>> [...] >>>> A large P-value means nothing more than needing more data. No >>>> conclusion is possible. Please read the classic paper Absence of >>>> Evidence is not Evidence for Absence. >>> > [...] >> >> It's real. Full text is available to all: >> http://www.bmj.com/cgi/content/full/311/7003/485 > > The quotation is attributed to the late Carl Sagan who > seemed to have used it as a strawman argument , see > http://oyhus.no/AbsenceOfEvidence.html.
This citation of Sagan, and the link therein to Sagan quotes: http://en.wikiquote.org/wiki/Carl_Sagan are interesting, as far as they go. However, I disagree with the proof ("by conditional probability") that absence of evidence is evidence of absence. Definition 1 is disputable. But, whether one agrees with it or not, Definition 2 does not correspond to my interpretation of "absence of evidence". If A is evidence for B (in terms of P(B|A) etc.), this means that if we *know* that A is the case, or that not-A is the case, then we can say something about P(B). But "absence of evidence", in my interpretation (which I believe is right for the statistical context of "non-significant P-values"), means that we do not know about A: we do not have enough information. That proof needs to be discussed in terms of the available evidence for A! The proof is, basically, given in terms of a 2-valued logic where every term is either TRUE or FALSE. In the real world we have at least a third possible value: UNKNOWN (or, as R would put it, NA). Even if you accept (Definition 1) that "A is evidence for B" == P(B|A) > P(B|not-A) what can you possibly say about P(B|NA) (other than that it is NA itself)? Best wishes to all, Ted. -------------------------------------------------------------------- E-Mail: (Ted Harding) <[EMAIL PROTECTED]> Fax-to-email: +44 (0)870 094 0861 Date: 13-Jul-08 Time: 21:59:16 ------------------------------ XFMail ------------------------------ ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
