On Jun 14, 2011, at 08:13 , Prof Brian Ripley wrote: > I presume you intended 'type' and 'fragment' to be factors (see below). Such > a model would fit exactly. The additive model > >> model <- glm(y ~ fragment+type, binomial) > > is only modestly over-dispersed, and shows that 'fragment' has zero effect. > Not 'a negligible effect', but no effect. So something really odd is going > on: is this an exercise with artificial data? > Otherwise you need to explain the exact balance between the two 'fragments' > (each fragment has exactly 1/4 success) and your assumption of independent > binomial sampling cannot be true.
Also note that success+failure is exactly 102 in fragment 1 and 105 in fragment 2, as is the sum of the successes for each fragment (of course it has to to make exactly 1/4). It is rather easy to suspect that it is actually a 0/1 coding of the type (as in "tick exactly one box"), and not independent binomial data. -- Peter Dalgaard Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
