I fit the model fit<-lm(thresh~cond*Ne) where thresh is the reponse cond is a factor with levels a, b, and c Ne is a continuous indep var I think of this full model as having three lines: thresh as a function of Ne for each condition. Thus we have slopea, slopeb, slopec, inta, intb, intc. lm output my params ------------------------- (Intercept) inta condb intb - inta condc intc - inta Ne slopea condb:Ne slopeb - slopea condc:Ne slopec - slopea This parametrization automatically compares inta vs intb inta vs intc But how to get test for diff of intb vs intc? Same question for slope: I already have slopea vs slopeb slopea vs slopec But how to get test for diff of slopeb vs slopec? Thanks for any help. Bill Simpson -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._
Bill Simpson <wsi at gcal.ac.uk> writes:> This parametrization automatically compares > inta vs intb > inta vs intc > But how to get test for diff of intb vs intc? > > Same question for slope: I already have > slopea vs slopeb > slopea vs slopec > But how to get test for diff of slopeb vs slopec?This is the sort of thing relevel() is for. -- O__ ---- Peter Dalgaard Blegdamsvej 3 c/ /'_ --- Dept. of Biostatistics 2200 Cph. N (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907 -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._
> > This is the sort of thing relevel() is for.Yes, that does the trick--thanks very much. Bill -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._
Dear Bill, At 11:58 AM 08/10/2001 +0100, Bill Simpson wrote:>I fit the model > >fit<-lm(thresh~cond*Ne) >where >thresh is the reponse >cond is a factor with levels a, b, and c >Ne is a continuous indep var > >I think of this full model as having three lines: thresh as a function of >Ne for each condition. Thus we have slopea, slopeb, slopec, inta, intb, >intc. >lm output my params >------------------------- >(Intercept) inta >condb intb - inta >condc intc - inta >Ne slopea >condb:Ne slopeb - slopea >condc:Ne slopec - slopea > >This parametrization automatically compares >inta vs intb >inta vs intc >But how to get test for diff of intb vs intc? > >Same question for slope: I already have >slopea vs slopeb >slopea vs slopec >But how to get test for diff of slopeb vs slopec? > >Thanks for any help.Using the covariance matrix for the coefficients and the coefficient-estimates, you can test the linear hypotheses that condb - condc = 0, and condb:Ne - condc:Ne = 0. The linear.hypothesis function in the car package does this, but it's also not hard to work directly with the estimates and their covariance matrix. An alternative (producing the same test) is to refit the model, reparametrizing to set a different baseline category for the factor. Finally, by doing these additional tests, you're using up more df than you have for comparisons among slopes and among intercepts. You might consider adjusting the tests for simultaneous inference. John ----------------------------------------------------- John Fox Department of Sociology McMaster University Hamilton, Ontario, Canada L8S 4M4 email: jfox at mcmaster.ca phone: 905-525-9140x23604 web: www.socsci.mcmaster.ca/jfox ----------------------------------------------------- -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._