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 [EMAIL PROTECTED] 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.html