From: Spencer Graves <spencer.graves_at_pdf.com>

Date: Wed 06 Jul 2005 - 07:28:58 EST

Date: Wed 06 Jul 2005 - 07:28:58 EST

I saw a standard overdispersed binomial. In particular, I saw NO evidence of saturation at 0.5 or anything below 1. I did the following:

tmp$N <- tmp$yes+tmp$no

with(tmp, plot(x, yes))

with(tmp, plot(x, yes/N))

tmp.glm <- glm(cbind(yes,no) ~ x, data = tmp, family = binomial(link
=logit))

tmp.glmq <- glm(cbind(yes,no) ~ x, data = tmp, family =
quasibinomial(link =logit))

summary(tmp.glm)

summary(tmp.glmq)

plot(tmp.glm)

plot(tmp.glmq)

# Test the statistical significance of the "Dispersion" parameter pchisq(summary(tmp.glmq)$dispersion*12, 12, lower=FALSE)

hope this helps. spencer graves

Kevin J Emerson wrote:

> R-helpers,

*>
**> I have a question about logistic regressions.
**>
**> Consider a case where you have binary data that reaches an asymptote
**> that is not 1, maybe its 0.5. Can I still use a logistic regression to
**> fit a curve to this data? If so, how can I do this in R. As far as I
**> can figure out, using a logit link function assumes that the asymptote
**> is at y = 1.
**>
**> An example. Consider the following data:
**>
**> "tmp" <-
**> structure(list(x = c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
**> 14), yes = c(0, 0, 0, 2, 1, 14, 24, 15, 23, 18, 22, 20, 14, 17
**> ), no = c(94, 101, 95, 80, 81, 63, 51, 56, 30, 38, 31, 18, 21,
**> 20)), .Names = c("x", "yes", "no"), row.names = c("1", "2", "3",
**> "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14"), class =
**> "data.frame")
**>
**> where x is the independent variable, and yes and no are counts of
**> events. plotting the data you can see that the data seem to reach an
**> asymptote at around y=0.5. using glm to fit a logistic regression it is
**> easily seen that it does not fit well.
**>
**> tmp.glm <- glm(cbind(yes,no) ~ x, data = tmp, family = binomial(link =
**> logit))
**> plot(tmp.glm$fitted, type = "l", ylim = c(0,1))
**> par(new=T)
**> plot(tmp$yes / (tmp$yes + tmp$no), ylim = c(0,1))
**>
**> Any suggestions would be greatly appreciated.
**>
**> Cheers,
**> Kevin
**>
*

-- 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 Wed Jul 06 07:34:19 2005

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