step.gam is a tricky function to use correctly. You will need to consult the original documentation (in Chambers & Hastie ca 1992) or ask the package author for help.
BTW, it uses loess not lowess. On Wed, 12 Oct 2005 [EMAIL PROTECTED] wrote: > > Hi, > > I'm working with step.gam in gam package. I'm interested both in spline and > lowess functions and when I define all the models that I'm interested in I get > something like that: > >> gam.object.ALC<-gam(X143S~ALC,data=dane,family=binomial) >> > step.gam.ALC<-step.gam(gam.object.ALC,scope=list("ALC"=~1+ALC+s(ALC,2)+s(ALC,3)+s(ALC,4)+s(ALC,6)+s(ALC,8)+lo(ALC,degree=1,span=.5)+lo(ALC,degree=2,span=.5)+lo(ALC,degree=1,span=.25)+lo(ALC,degree=2,span=.25))) > Start: X143S ~ ALC; AIC= 104.0815 > Trial: X143S ~ 1; AIC= 111.1054 > Trial: X143S ~ s(ALC, 2); AIC= 103.3325 > Step : X143S ~ s(ALC, 2) ; AIC= 103.3325 > > Trial: X143S ~ s(ALC, 3); AIC= 102.9598 > Step : X143S ~ s(ALC, 3) ; AIC= 102.9598 > > Trial: X143S ~ s(ALC, 4); AIC= 102.2103 > Step : X143S ~ s(ALC, 4) ; AIC= 102.2103 > > Trial: X143S ~ s(ALC, 6); AIC= 102.4548 > > I have impression that the algorithm stops when the next trial gives higher > AIC > without examining further functions. When I deleted some of the spline > functions > that were worse than s(ALC,4) I got: > > > > step.gam.ALC<-step.gam(gam.object.ALC,scope=list("ALC"=~1+ALC++s(ALC,4)+lo(ALC,degree=1,span=.5)+lo(ALC,degree=2,span=.5)+lo(ALC,degree=1,span=.25)+lo(ALC,degree=2,span=.25))) > Start: X143S ~ ALC; AIC= 104.0815 > Trial: X143S ~ 1; AIC= 111.1054 > Trial: X143S ~ s(ALC, 4); AIC= 102.2103 > Step : X143S ~ s(ALC, 4) ; AIC= 102.2103 > > Trial: X143S ~ lo(ALC, degree = 1, span = 0.5); AIC= 99.8127 > Step : X143S ~ lo(ALC, degree = 1, span = 0.5) ; AIC= 99.8127 > > Trial: X143S ~ lo(ALC, degree = 2, span = 0.5); AIC= 100.5275 > > Lowess turned out to be better in this situation. Is there any way to examine > all the models without stopping when AIC is higher in the next trial? Or maybe > manual handling is the only solution? > > thanks for help in advance > > Agnieszka Strzelczak > > ______________________________________________ > R-help@stat.math.ethz.ch mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html > -- 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@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html