From: Spencer Graves <spencer.graves_at_pdf.com>

Date: Tue 12 Jul 2005 - 03:39:37 EST

*>
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*>
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> You can always extract the `exact' p-value from the "summary.lm" object or

*> you can compute it by hand via
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*> pf(39540, df1 = 1, df2 = 7025, lower.tail = FALSE)
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*> For all practical purposes, the above means that the p-value is 0.
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*> I guess you are on a 32-bit machine, then it also means that the p-value
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*> is smaller than the Machine epsilon
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*> .Machine$double.eps
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*>
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*> So if you want to report the p-value somewhere, I think R's output should
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*> be more than precise enough. If you want to compute some other values that
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*> depend on such a p-value, then it is probably wiser to compute on a log
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*> scale, i.e. instead
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*> pf(70, df1 = 1, df2 = 7025, lower.tail = FALSE)
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*> use
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*> pf(70, df1 = 1, df2 = 7025, lower.tail = FALSE, log.p = TRUE)
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*>
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*> However, don't expect to be able to evaluate it at such extreme values
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*> such as 39540.
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*> Z
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*>
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Date: Tue 12 Jul 2005 - 03:39:37 EST

> pf(39540, 1, 7025, lower.tail=FALSE, log.p=TRUE) [1] -Inf

> pchisq(39540, 1, lower.tail=FALSE, log.p=TRUE) [1] -19775.52

I'm confident that many violations of assumptions would likely be more important than the differences between "p-value: < 2.2e-16" and these other two answers. However, I have also used numbers like exp(-19775.52) to guestimate relative degrees of plausibility for different alternatives. That doesn't mean they are right, only the best I can get with the available resources.

spencer graves

Achim Zeileis wrote:

> On Mon, 11 Jul 2005, S.O. Nyangoma wrote:

*>
**>
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>> Hi there, >> If I do an lm, I get p-vlues as >> >> p-value: < 2.2e-16 >> >>This is obtained from F =39540 with df1 = 1, df2 = 7025. >> >> Suppose am interested in exact value such as >> >> p-value = 1.6e-16 (note = and not <) >> >> How do I go about it?

> You can always extract the `exact' p-value from the "summary.lm" object or

-- Spencer Graves, PhD Senior Development Engineer PDF Solutions, Inc. 333 West San Carlos Street Suite 700 San Jose, CA 95110, USA spencer.graves@pdf.com www.pdf.com <http://www.pdf.com> Tel: 408-938-4420 Fax: 408-280-7915 ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.htmlReceived on Tue Jul 12 03:49:11 2005

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