[R] R interpretation

dan kumpik daniel.kumpik at physiol.ox.ac.uk
Mon Jan 22 21:10:24 CET 2007


I am new to R (and not really a stats expert) and am having trouble 
interpreting its output. I am running a human learning experiment, with 
6 scores per subject in both the pretest and the posttest. I believe I 
have fitted the correct model for my data- a mixed-effects design, with 
subject as a random factor and session (pre vs post) nested within group 
(trained vs control).

I am confused about the output. The summary command gives me this table:

  > D.lme<- lme(score~GROUP/session, random=~1|subject, data=ILD4L )
  > summary(D.lme)

Linear mixed-effects model fit by REML
   Data: ILD4L
    Subset: EXP == "F"
          AIC       BIC   logLik
    -63.69801 -45.09881 37.84900

Random effects:
   Formula: ~1 | subject
          (Intercept)  Residual
StdDev:   0.1032511 0.1727145

Fixed effects: score ~ GROUP/session
                           Value  Std.Error  DF   t-value p-value
(Intercept)         0.10252778 0.05104328 152  2.008644  0.0463
GROUPT              0.09545347 0.06752391  12  1.413625  0.1829
GROUPC:sessionpost -0.00441389 0.04070919 152 -0.108425  0.9138
GROUPT:sessionpost -0.23586042 0.03525520 152 -6.690090  0.0000
                     (Intr) GROUPT GROUPC
GROUPT             -0.756
GROUPC:sessionpost -0.399  0.301
GROUPT:sessionpost  0.000 -0.261  0.000

Standardized Within-Group Residuals:
          Min          Q1         Med          Q3         Max
-2.66977386 -0.52935645 -0.08616759  0.57215015  3.26532101

Number of Observations: 168
Number of Groups: 14

I believe the fixed-effects section of this output to be telling me that
my model intercept (which I assume to be the control group pretest?) is
significantly different from 0, and that GROUPT (i.e. the trained group)
does not differ significantly from the intercept- therefore no pretest
difference between groups?
	The next line is, I believe showing that the GROUPC x sessionpost
interaction (i.e., control posttest scores?) is not significantly
different from the intercept (i.e. control pretest scores). Finally, I
am interpreting the final line as indicating that the GROUPT x
sessionpost interaction (ie, trained posttest scores?) is significantly
different from the trained pretest scores (GROUPT). A treatment contrast 
that I would like to apply would be for Control-post vs Trained-post, to 
see if the groups differ after training, but I'm not sure how to do 
this- and I feel I am probably overcomplicating the matter.

I am confused about how to report this output in my publication. For 
instance, what should I be reporting for df? Those found on the output 
of the anova table?

Would it be possible to look through this for me and indicate how to
interpret the R output, and also how I should be reporting this? 
Apologies for asking such basic questions, but I would like to start 
using R more regularly and to make sure I am doing so correctly.

Many thanks,


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