Thank you for your suggestions, stats experts. Much appreciated. I still haven't got what I wanted but someone suggested looking into contrasts and this is looking worth trying http://finzi.psych.upenn.edu/R/library/gmodels/html/fit.contrast.html
Regards, Yusuke -----Original Message----- From: Peter Ehlers [mailto:ehl...@ucalgary.ca] Sent: Saturday, 2 April 2011 1:35 AM To: Yusuke Fukuda Cc: 'Bert Gunter'; r-help@r-project.org Subject: Re: [R] ANCOVA for linear regressions without intercept See inline. On 2011-03-31 22:22, Yusuke Fukuda wrote: > Thanks Bert. > > I have read "?formula" again and again, and I'm still struggling; > >> lm(body_length ~ head_length-1) > > This removes intercept from each individual regression (for male, female, > unknown). > > When they are taken together, > >> lm(body_length ~ sex*head_length) > > This shows differences in slopes and intercepts between the regressions (but > I want to compare the slopes of the regressions WITHOUT intercepts). > > If I put > >> lm(body_length ~ sex:head_length-1) > > This shows slopes for each sex without intercepts, but NOT differences in the > slope between the regressions. You probably want: lm(body_length ~ head_length + sex:head_length-1) or, in short form: lm(body_length ~ head_length/sex-1) You might then compare the model 'without intercepts' (i.e. with intercepts forced to zero) with a model that includes intercepts. If the intercepts turn out to be significantly nonzero, what will you do? Peter Ehlers > > I also tried > >> lm(body_length ~ sex*head_length-1) >> lm(body_length ~ sex*head_length-sex-1) > > But none of them worked. > > Would anyone be able to help me? All I want to do is to compare the slopes of > three linear regressions that go through the origin (0,0) so that I can say > if their difference is significant or not. > > Thanks for your help. > > > > ________________________________________ > From: Bert Gunter [mailto:gunter.ber...@gene.com] > Sent: Friday, 1 April 2011 12:56 AM > To: Yusuke Fukuda > Cc: r-help@r-project.org > Subject: Re: [R] ANCOVA for linear regressions without intercept > > If you haven't already received an answer, a careful reading of > > ?formula > > will provide it. > > -- Bert > On Wed, Mar 30, 2011 at 11:42 PM, Yusuke Fukuda<yusuke.fuk...@nt.gov.au> > wrote: > > Hello R experts > > I have two linear regressions for sexes (Male, Female, Unknown). All have a > good correlation between body length (response variable) and head length > (explanatory variable). I know it is not recommended, but for a good > practical reason (the purpose of study is to find a single conversion factor > from head length to body length), the regressions need to go through the > origin (0 intercept). > > Is it possible to do ANCOVA for these regressions without intercepts? When I > do > > summary(lm(body length ~ sex*head length)) > > this will include the intercepts as below > > > Coefficients: > Estimate Std. Error t value Pr(>|t|) > (Intercept) -6.49697 1.68497 -3.856 0.000118 *** > sexMale -9.39340 1.97760 -4.750 2.14e-06 *** > sexUnknown -1.33791 2.35453 -0.568 0.569927 > head_length 7.12307 0.05503 129.443< 2e-16 *** > sexMale:head_length 0.31631 0.06246 5.064 4.37e-07 *** > sexUnknown:head_length 0.19937 0.07022 2.839 0.004556 ** > --- > > Is there any way I can remove the intercepts so that I can simply compare the > slopes with no intercept taken into account? > > Thanks for help in advance. > > Yusuke Fukuda > > ______________________________________________ > 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.
