On Wed, 2010-08-18 at 08:51 +0100, Gavin Simpson wrote: > On Wed, 2010-08-18 at 05:42 +0800, elaine kuo wrote: > > Thank you. > > Most of the answers solved the puzzles. <snip /> > > => Please explain why fitted lm is better for GLM. > > Seriously? A GLM specified as glm(...., family = gaussian) is the linear > model that you'd get with lm(). lm() fits the model far more efficiently > than glm(). The code you showed specifically used 'family = gaussian', > hence my comment.
Hmmm. Thinking some more, I might not have answered your (unstated) question. What do your mean by GLM? I mean the Generalized Linear Model, not the General Linear Model. The Generalized one allows for non-normal responses and different mean-variance relationships and is the GLM of Nelder and Wedderburn (1972, J. Royal Statistical Society, Series A, 135(3),370-384) and the monograph by McCullagh and Wedderburn (1989, Generalized Linear Models, Chapman & Hall/CRC). The R function glm() fits these kinds of model. The General Linear Model (IIRC) was the linking of linear regression and anova into a single entity. The R function lm() fits these kinds of models. The linear model is a special case of the Generalized Linear Model when the Gaussian error is used with the identity link function. Hence a Gaussian GLM (my GLM) with the identity link fitted by glm() will give the same results as lm(), but it will do so in a very inefficient manner. As this was what your code was doing I suggested using lm() instead. HTH G > > > But temp_ran is not in your model... > > > > > error in eval(expr, envir, enclos), 'temp_ran' not found > > > > > > When used properly (none of this datam.std$ business), subset > > will do > > what you want: > > > > > dd2 <- dredge(lm1, subset = X1) > > > dd2 > > Global model: lm(formula = y ~ ., data = Cement) > > --- > > Model selection table > > (Int) X X1 X2 X3 X4 k R.sq > > Adj.R.sq RSS > > 3 52.58 1.4680 0.6623 4 0.9787 > > 0.9744 57.90 > > > > > > => Please suggest how to define subset in my case > > How would I know? I still haven't seen your data. You seem to be > mistaken on what is and is not included in your model and you fitted it. > What hope do we have...? However, given the model 'mig.stds' from above > in this email: > > > mig.stds <-lm(SummerM_ratio ~ temp_max + evi_mean + topo_var + > > topo_mean + coast + Iso_index_0808, > > ## now tell R were to find the variables in formula > > data = datum.std) > > ## If you are fitting a Gaussian GLM it is better fitted with lm() > > If you want to consider dredged models containing temp_max, then you > would do > > dredge(mig.stds, subset = temp_max) > > If you want models that contain temp_max and coast, then you'd do > > dredge(mig.stds, subset = temp_max & coast) > > or > > dredge(mig.stds, fixed = ~ temp_max + coast) > > The bits you include in subset or fixed are the names of your variables > that you want in or out of the models. In your case, the names of the > variables as input into the model formula. With 'subset' you need to use > logical operators (and [&], or [|]) whilst with 'fixed' you can specify > a formula of variables that should be included or excluded in the same > way you'd write any R formula. > > But, now having been told this, please note that this is *all* discussed > on the ?dredge help page if you bother to read it. I've never used this > package, and, OK, I have used R for going on for 11 or 12 years now so > am used to reading help pages and understand the language a bit more you > perhaps do, but you do seem to be asking questions or running into > problems that are all covered by the help pages. > > > Finally, it would be highly appreciated to recommend any references of > > R for a beginner like me. > > Read the An Introduction to R manual that comes with R or that can be > downloaded from the R website/CRAN. Also look at the contributed > documentation section on the R website which contains numerous free > introductory guides. > > > Elaine > > HTH > > G > -- > %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% > 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. -- %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% 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.
