Either lm(y~ a-1 + a:x) or lm (y~ a + (a-1):x) or lm(y~ a+ a:(x-1)) -- Bert Gunter Genentech
-----Original Message----- From: bgunter Sent: Friday, May 16, 2008 8:39 AM To: Rolf Turner Subject: RE: [R] Making slope coefficients ``relative to 0''. lm(y ~ a + a:(x-1)) -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Rolf Turner Sent: Thursday, May 15, 2008 8:55 PM To: R-help forum Subject: [R] Making slope coefficients ``relative to 0''. I am interested in whether the slopes in a linear model are different from 0. I.e. I would like to obtain the slope estimates, and their standard errors, ``relative to 0'' for each group, rather than relative to some baseline. Explicitly I would like to write/represent the model as y = a_i + b_i*x + E i = 1, ..., K, where x is a continuous variate and i indexes groups (levels of a factor with K levels). The ``usual'' structure (using ``treatment contrasts'') gives y = a + a_i + b*x + b_i*x + E i = 2, ..., K. (So that b is the slope for the baseline group, and b_i measures how much the slope for group i differs from that for the baseline group. I can force the *intercepts* to be ``relative to 0'' by putting a ``-1'' into the formula: lm(y ~ g*x-1) But I don't really care about the intercepts; it's the slopes I'm interested in. And there doesn't seem to a way to do the thing equivalent to the ``-1'' trick for slopes. Or is there? There are of course several work-arounds. (E.g. calculate my b_i- hats and their standard errors from the information obtained from the usual model structure. Or set up my own dummy variable to regress upon. Easy enough, and I could do that.) I just wanted to know for sure that there wasn't a sexier way, using some aspect of the formula machinery with which I am not yet familiar. Thanks for any insights. cheers, Rolf Turner ###################################################################### Attention:\ This e-mail message is privileged and confid...{{dropped:9}} ______________________________________________ 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. ______________________________________________ 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.
