dropterm() is a tool for model building, not primarily for significance testing.
As the name suggests, it tells you what the effect would be were you to "drop" each *accessible* "term" in the model as it currently stands. By default it displays the effect on AIC of dropping each term, in turn, from the model. If you request them, though, it can also give you test statistics and significance probabilities. If there is an "A:B" interaction in the model, the main effects "A" or "B", if present, are not considered until a decision has been made on including "A:B". The meaning of "A:B" in a model is not absolute: it is conditional on which main effect terms you have there as well. This is one reason why the process is ordered in this way, but the main reason is the so-called 'marginality' issue. If you do ask for test statistics and significance probabilities, you get almost a SAS-style "Type III" anova table, with the important restriction noted above: you will not get main effect terms shown along with interactions. If you want the full SAS, uh, version, there are at least two possibilities. 1. Use SAS. 2. Use John Fox's Anova() function from the 'car' package, along with his excellent book, which should explain how to avoid shooting yourself in the foot over this. (This difference of opinion on what should sensibly be done, by the way, predates R by a long shot. My first exposure to it were with the very acrimonious disputes between Nelder and Kempthorne in the mid 70's. It has remained a cross-Atlantic dispute pretty well ever since, with the latest shot being the paper by Lee and Nelder in 2004. Curiously, the origin of the software can almost be determined by the view taken on this issue, with Genstat going one way and SAS, SPSS, ... the other. S-PLUS was a late comer...but I digress!) Bill Venables. Bill Venables CSIRO Laboratories PO Box 120, Cleveland, 4163 AUSTRALIA Office Phone (email preferred): +61 7 3826 7251 Fax (if absolutely necessary): +61 7 3826 7304 Mobile: +61 4 8819 4402 Home Phone: +61 7 3286 7700 mailto:[EMAIL PROTECTED] http://www.cmis.csiro.au/bill.venables/ -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of DaniWells Sent: Sunday, 10 February 2008 11:40 PM To: [email protected] Subject: [R] Do I need to use dropterm()?? Hello, I'm having some difficulty understanding the useage of the "dropterm()" function in the MASS library. What exactly does it do? I'm very new to R, so any pointers would be very helpful. I've read many definitions of what dropterm() does, but none seem to stick in my mind or click with me. I've coded everything fine for an interaction that runs as follows: two sets of data (one for North aspect, one for Southern Aspect) and have a logscale on the x axis, with survival on the y. After calculating my anova results i have all significant results (ie aspect = sig, logscale of sunlight = sig, and aspect:llight = sig). When i have all significant results in my ANOVA table, do i need dropterm(), or is that just to remove insignificant terms? Many thanks, Dani -- View this message in context: http://www.nabble.com/Do-I-need-to-use-dropterm%28%29---tp15396151p15396 151.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ [email protected] 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. ______________________________________________ [email protected] 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.
