[R] How to prevent inclusion of intercept in lme with interaction

[ONKELINX, Thierry]

Dear Dieter,

With t*treat the model allows for a different slope AND a different
intercept for each treatment. If you only want different slopes and all
intercepts equal to 0, then t:treat - 1 or t + t:treat - 1 is the model
you are looking for.

HTH,

Thierry


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium 
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be 
www.inbo.be 

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens Dieter Menne
Verzonden: woensdag 1 april 2009 14:11
Aan: r-help at stat.math.ethz.ch
Onderwerp: [R] How to prevent inclusion of intercept in lme with
interaction

Dear friends of lme,

After so many year with lme, I feel ashamed that I cannot get this to
work.
Maybe it's a syntax problem, but possibly a lack of understanding.

We have growth curves of new dental bone that can well be modeled by a
linear growth curve, for two different treatments and several subjects
as
random parameter. By definition, newbone is zero at t=0, so I tried to
force
the curve through 0. In Pinheiro/Bates, this is done by including the -1
term, and it works well when treatment is not included (newbone~t-1),
but
seems to have no effect in (newone ~ t*treat-1).

What's wrong? Some bracket missing? I tried a few variants.

Dieter Menne
#--------------------------------
library(nlme)
library(lattice)
# Generated data
set.seed(4711)
subject = as.factor(letters[1:5])
varslope = rnorm(length(subject),0,0.02)
cslope = c (0.1,0.15)
grd = expand.grid(t=seq(5,15,by=5), 
     subject=subject,treat=c("contr","test"))
grd$slope = varslope[grd$subject] + cslope[grd$treat]
grd$newbone = grd$slope*grd$t+rnorm(nrow(grd),0,0.2)

xyplot(newbone~t|treat,groups=subject,data=grd,
   type="l",xlim=c(0,20),ylim=c(0,3))

# With intercept
grd.lme1 = lme(newbone~t*treat,data=grd,random=~1|subject)
grd$pred1 = predict(grd.lme1,level=0)
summary(grd.lme1)

# How go force intercept = 0 ???
grd.lme0 = lme(newbone~t*treat-1,data=grd,random=~1|subject)
grd$pred0 = predict(grd.lme0,level=0)
summary(grd.lme0)
# Gives true,
all.equal(grd$pred1,grd$pred0)

# Everything as expected without treat
grd.lme2 = lme(newbone~t,data=grd,random=~1|subject)
grd$pred2 = predict(grd.lme2,level=0)
summary(grd.lme2)

# Forced intercept = 0
grd.lme3 = lme(newbone~t-1,data=grd,random=~1|subject)
grd$pred3 = predict(grd.lme3,level=0)
summary(grd.lme3)
# As expected: not equal
all.equal(grd$pred2,grd$pred3)

#-------------------------------------------------------------------
R version 2.9.0 Under development (unstable) (2009-03-13 r48127) 
i386-pc-mingw32 

locale:
LC_COLLATE=German_Germany.1252;LC_CTYPE=German_Germany.1252;
LC_MONETARY=German_Germany.1252;LC_NUMERIC=C;LC_TIME=German_Germany.1252

attached base packages:
[1] stats     graphics  grDevices datasets  utils     methods   base


other attached packages:
[1] lattice_0.17-20 nlme_3.1-90    

loaded via a namespace (and not attached):
[1] grid_2.9.0  tools_2.9.0
>

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