On Tue, 2010-08-17 at 17:12 +0100, Xin__ Li wrote: > Hi All: > the package "MuMIn" can be used to select the model based on AIC or AICc. > The code is as follows: > > data(Cement) > lm1 <- lm(y ~ ., data = Cement) > > dd <- dredge(lm1,rank="AIC") > print(dd) > > If I want to select the model by BIC, what code do I need to use?
?dredge explains this a bit. The key is to write your own function to compute BIC and use that instead. I haven't looked into this in any detail, but the following seems to work: BIC <- function(object, k, ...) return(AIC(object, k = k, ...)) data(Cement) lm1 <- lm(y ~ ., data = Cement) dd <- dredge(lm1, rank = "BIC", k = log(NROW(Cement))) dd The BIC function in this case works because dredge passes on the '...' to the rank function. Which actually means, all you need to do is: dd2 <- dredge(lm1, rank = "AIC", k = log(NROW(Cement))) dd2 So in this case, no need for the BIC function, but I left it in in case you ever want to have your own ranking function. > And when > to select the best model based on AIC, what the differences between the > function "dredge" in package"MuMIn" and the function "stepAIC" in package > "MASS" I'm not going there. This really depends on why you are doing these data dredging steps in first place; prediction or inference. And that is beyond the bounds of this list (for me at least). As for differences, dredge() fits all combination of models given a set of predictors, stepAIC() performs stepwise (forwards, backwards or both) selection within an upper and lower scope for the models. HTH G > > Many thanks > > best wishes > > XIN LI > > [[alternative HTML version deleted]] > > ______________________________________________ > 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. -- %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% Dr. Gavin Simpson [t] +44 (0)20 7679 0522 ECRC, UCL Geography, [f] +44 (0)20 7679 0565 Pearson Building, [e] gavin.simpsonATNOSPAMucl.ac.uk Gower Street, London [w] http://www.ucl.ac.uk/~ucfagls/ UK. WC1E 6BT. [w] http://www.freshwaters.org.uk %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% ______________________________________________ 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.
