I think you are looking for y ~ x + x:factor E.g.
library(car) lm(repwt ~ repht + repht:sex, data=Davis)
Coefficients: (Intercept) repht repht:sexM -59.30865 0.71412 0.05694 where the third term is the difference in slope between males and females.
lm(repwt ~ repht:sex, data=Davis)
Coefficients: (Intercept) repht:sexF repht:sexM -59.3086 0.7141 0.7711 for separately reported slopes.If you want to constrain the intercept, fit with and without and take the better fit (or look into package nnls, but that would be overkill here).
On Wed, 28 Nov 2007, [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 ---------------------------------------------------------------------------------------------------------------------- NIVAs hovedkontor har flyttet til nye lokaler i CIENS - Forskningssenter for miljø og samfunn; Gaustadalléen 21, 0349 Oslo. Meld deg på vårt nyhetsbrev på www.niva.no ______________________________________________ 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.
-- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595
______________________________________________ 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.
