Path: utzoo!attcan!uunet!mailrus!purdue!mentor.cc.purdue.edu!l.cc.purdue.edu!cik
From: cik@l.cc.purdue.edu (Herman Rubin)
Newsgroups: news.groups
Subject: Re: RESULTS of OPINION POLL on Renaming Newsgroups
Summary: Self-selection destroys the validity of any poll
Keywords: little, knowledge, dangerous, thing
Message-ID: <1740@l.cc.purdue.edu>
Date: 24 Nov 89 13:41:27 GMT
References: <971@csisles.Bristol.AC.UK> <972@csisles.Bristol.AC.UK> <1199@csisles.Bristol.AC.UK>
Organization: Purdue University Statistics Department
Lines: 25

In article <1199@csisles.Bristol.AC.UK>, mackeown@CompSci.Bristol.AC.UK (W. Mackeown) writes:
			...........................

> This poll is simply a case of binomial sampling without replacement from a
> large but finite population. Since I received 52 replies, the statistics for
> these poll results are such that I can say that there is only a 1 in 20 chance
> that the true opinion of the Usenet news.groups readership is not within 6.47%
> of 66% in favour of renaming.

For this to be the case without strong, usually untenable, assumptions,
requires that the probability of any set of k individuals being included
must be the same as that for any other set of k individuals.  [Most polls
do something better, namely, stratified random sampling, but the basic idea
is the same.]  The problem is whether the individuals responding voluntarily
are representative, and this is usually false.

Any inference based on voluntary responses is so fraught with danger as to
be totally untrustworthy without making very strong assumptions which are
extremely unlikely to be reasonable.  There are Bayesian ways of handling
the problem, but these are very sensitive to the assumptions, and the 
conclusion may very well depend more on the assumptions than on the data.
-- 
Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907
Phone: (317)494-6054
hrubin@l.cc.purdue.edu (Internet, bitnet, UUCP)