[EMAIL PROTECTED] wrote: > Hi group, > > I have this type of data > x(predictor), y(response), factor (grouping x into many groups, with 6-20 > obs/group) > > I want to fit a linear regression with one common intercept. 'factor' > should only modify the slopes, not the intercept. The intercept is expected > to be >0. > > If I use > y~ x + factor, I get a different intercept for each factor level, but one > slope only > > if I use > y~ x * factor, I get the interaction term I want, but the intercept is not > kept constant. > Also, if I constrain teh intercept in the regression model (y~a+x*factor), > I get estimates both for slope and intercept of each factor level. > > Robert > > > > You seem to be looking for the colon operator. In R, unlike certain other statistical packages, the star implies inclusion of main effects: a*b is a + b + a:b. There is some trickery about when you get factors contrast coded in interaction terms (as far as I remember x:factor and x+x:factor are two different parametrizations of the same model), but you should be able to find that out by a little experimenting.
-- O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 ______________________________________________ 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.
