On Thu, 2008-02-07 at 11:16 -0400, tyler wrote: > On Thu, Feb 07, 2008 at 02:36:58PM +0000, Gavin Simpson wrote: > > > > But I'm not sure this matters much. If you use the formula interface to > > lda(), factors get expanded to the dummy variables Tyler is talking > > about. But of course, a factor with two levels 0/1 doesn't need much > > manipulation as you only need a single dummy variable to represent its > > two states: > > > > Thanks, Gavin! > > R's formula interface if very powerful, and I'm just starting to > understand how to take full advantage of it. > > > You might want to standardise your exp variables to zero mean and unit > > variance prior to doing the lda so that all variables carry the same > > weight, if you have mixtures of numeric (continuous) variables and > > binary ones. > > This is the part I was unsure of. If you have a categorical > explanatory variable with five levels, you can turn it into four dummy > variables, which you then standardize. Does the original variable end > up getting four times the weight of a single numerical variable?
I have no idea Tyler. I would take great heed in the warning about using only continuous variables in lda() - the author(s) of that function certainly know what they are talking about! One is probably violating some of the underlying assumptions of the method using binary/categorical data. There are better classification tools available than LDA for mixed data like this, such as a classification tree, which is easy to interpret (always good for ecologists ;-) or some of the bagging, boosting or random forest algorithms also spring to mind. There has been quite a lot on these techniques in the ecological literature in the past 3-4 years. HTH G > > Cheers, > > Tyler > -- %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% 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.
