On 27 Oct 2000 07:26:43 -0700, [EMAIL PROTECTED] (Wolfgang
Rolke) wrote:
< snip, about polling ... >
> A very nice
> discussion of the issues involved can be found at
> http://slate.msn.com/framegame/entries/00-10-26_92147.asp . An explanation of the
> methodology of the Gallup tracking poll is at
> http://www.gallup.com/poll/faq/faq000101.asp , where they say that the +-4% is
> for a 95% confidence interval. I would like to pose another question, though,
> namely what a reader should make of all these polls. There is of course the
> additional problem of the horse race question: if Bush's percentage is estimated
> at 46%+-4% and Gore gets 44%+-4%, than in fact Bush is 2% ahead of Gore, but this
> difference has an error of +-8% and so is clearly not significant. Now if a poll
< snip, rest >
No, you have it wrong. Think about the case where the two
candidates/options add exactly to 100% -- the "4% error" is exactly
the same 4% for both, since one goes down exactly as much as the other
goes up. This is "correlation" among the proportions. With a few
percent as "Missing" or "Nader", the Gallup correlation of support for
major candidates is not quite -1.0, but it is close enough.
Oh, by the way, when you do have error terms to add, it is almost
always the squared values, "variances," that get summed. So two
Errors of 1.0 add up to one error of 1.41, since that is the square
root of (1-squared+ 1-squared).
--
Rich Ulrich, [EMAIL PROTECTED]
http://www.pitt.edu/~wpilib/index.html
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