On Sat, 2007-11-17 at 09:51 -0800, Ben Bolker wrote: > > > Ross Boylan wrote: > > > > I have a data frame "reading" that includes a logical variable "OLT" > > along with response variable "Reading" and predictor "True" (BOTH are > > numeric variables; it's "True" as in the true value). > > > > When I suppress the intercept, model.matrix gives me OLTTRUE and > > OLTFALSE columns. Why? Can I do anything to prevent it? > > > > > > I guess I don't understand the question -- this seems to be the > right behavior ... if you fitted the model Reading~OLT-1, you > need two coefficients, one to predict the value of cases where > OLT=FALSE and one to predict the value where OLT=TRUE. > You can parameterize the model as (value when FALSE, > difference between FALSE and TRUE) or (value when TRUE, > difference between TRUE and FALSE) or (value when TRUE, > value when FALSE) -- but however you do it you'll need two > variables in the model matrix -- right? Adding a continuous > predictor shouldn't matter. What I want is is a model that fits one coefficient. When OLT is false, it predicts 0; when OLT is true, it predicts the value of the coefficient.
I'm computing a likelihood from 2 separate design matrices and some other terms that don't appear in the formulae at all (those "other terms" are the intercepts in the sense appropriate for the model). I thought the -1 trick might work by analogy with the behavior for continuous predictors. > > If you don't want the extra column you can always drop > it with mm[,-1] ... Thanks. That's what I did. > > Ben Bolker > -- Ross Boylan wk: (415) 514-8146 185 Berry St #5700 [EMAIL PROTECTED] Dept of Epidemiology and Biostatistics fax: (415) 514-8150 University of California, San Francisco San Francisco, CA 94107-1739 hm: (415) 550-1062 ______________________________________________ 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.
