[R] Model simplification using anova()

From: ChCh <jmo101_at_student.canterbury.ac.nz>
Date: Tue, 03 Jun 2008 16:21:58 -0700 (PDT)

Hello all,

I've become confused by the output produced by a call to anova(model1,model2). First a brief background. My model used to predict final tree height is summarised here:

                            Df Sum Sq Mean Sq F value    Pr(>F)    
Treatment              2   748.35   374.17 21.3096   7.123e-06 ***
HeightInitial            1   0.31      0.31     0.0178    0.89519    
DiameterInitial         1   0.52      0.52     0.0298    0.86460    
Frost                     1   38.29    38.29    2.1807    0.15392    
HeightInitial:Frost    1   85.83     85.83   4.8882    0.03774 *  
DiameterInitial:Frost 1   97.90    97.90    5.5754    0.02749 *  
Residuals                22 386.30   17.56                      
---

Based on this, I should not remove either of the interaction terms, so I
turned my attention to the main factors. Based on p-values, I removed
HeightInitial and used a call to anova(model1,model2) to see if this
resulted in a weaker model.  Here is the output:

Model 1: HeightFinal ~ Treatment + HeightInitial + DiameterInitial + Frost +
HeightInitial:Frost + DiameterInitial:Frost

Model 2: AbsoluteDiameterDiff ~ Treatment + DiameterInitial + Frost +
HeightInitial:Frost + DiameterInitial:Frost

  Res.Df   RSS Df Sum of Sq F Pr(>F)
1     22 386.3                      
2     22 386.3  0 -1.08e-12        

This is not the output that I'm used to seeing.  Typically, a pvalue would
be provided to suggest if the models differ significantly from one another.  

Can anyone explain why there's no pvalue in this situation and whether or
not removing HeightInitial was justified.

Thanks,
Jacob Freel
-- 
View this message in context: http://www.nabble.com/Model-simplification-using-anova%28%29-tp17636012p17636012.html
Sent from the R help mailing list archive at Nabble.com.

______________________________________________
R-help_at_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.
Received on Wed 04 Jun 2008 - 01:09:47 GMT

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.2.0, at Wed 04 Jun 2008 - 08:30:36 GMT.

Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-help. Please read the posting guide before posting to the list.

list of date sections of archive