The important element in the fit object is named coefficients not coef. Sometimes the partial matching can cause confusion when it tries to help. In your case it creates a copy of the coefficients, changes the 2nd value, then creates a new component to fit called coef with these values (not changing the original coefficients). Since the predict, print, summary, etc. functions use the coefficients element rather than your new coef element, your changes are silently ignored.
One option is to spell out coefficients fully to replace that (though that could end up breaking other things), another possible option is to create the model using offsets (but I'm not sure how that would work in this case). Hope this helps, -- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare [EMAIL PROTECTED] (801) 408-8111 > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of > [EMAIL PROTECTED] > Sent: Tuesday, August 28, 2007 7:33 AM > To: r-help@stat.math.ethz.ch > Subject: [R] Forcing coefficients in lm object > > Dear all, > > I would like to use predict.lm() with an existing lm object > but with new arbitrary coefficients. I modify 'fit$coef' (see > example below) "by hand" but the actual model in 'fit' used > for prediction does not seem to be altered (although fit$coef is!). > > Can anyone please help me do this properly? > > Thanks in advance, > > Jérémie > > > > > dat <- data.frame(y=c(0,25,32,15), x=as.factor(c(1,1,2,2))) fit <- > > lm(y ~ x, data=dat) fit > > Call: > lm(formula = y ~ x, data = dat) > > Coefficients: > (Intercept) x2 > 12.5 11.0 > > > fit$coef[[2]] <- 100 > > dat.new <- data.frame(x=as.factor(c(1,2,1,2))) > > predict.lm(fit, dat.new) > 1 2 3 4 > 12.5 23.5 12.5 23.5 > > fit > > Call: > lm(formula = y ~ x, data = dat) > > Coefficients: > (Intercept) x2 > 12.5 11.0 > > > fit$coef > (Intercept) x2 > 12.5 100.0 > > > > > > Jérémie Lebrec > Dept. of Medical Statistics and Bioinformatics Leiden > University Medical Center Postzone S-05-P P.O. Box 9600 2300 > RC Leiden The Netherlands [EMAIL PROTECTED] > > ______________________________________________ > R-help@stat.math.ethz.ch 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. > ______________________________________________ R-help@stat.math.ethz.ch 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.
