[R] Constructing groupedData objects in nlme - a little problem

From: Toby Marthews <Toby.Marthews_at_lsce.ipsl.fr>
Date: Wed, 04 Jun 2008 16:01:48 +0200 (CEST)


Dear R-help,

I am trying to create groupedData objects using the nlme library. I'm missing something basic, I know:

Here is the first example in ch.1 of Pinheiro & Bates (2000):

library(nlme)
x2=Rail$travel;x1=Rail$Rail;eg1=data.frame(x1,x2);eg1gd=Rail print(eg1gd)
x11();print(plot(eg1gd))
femodel=lm(x2~x1-1,data=eg1gd)
print(femodel$coefficients)
  Result:

       x12 x15 x11 x16 x13 x14   31.66667 50.00000 54.00000 82.66667 84.66667 96.00000

...which works fine. This uses a built-in groupedData object called "Rail"
that is part of the nlme library.
  I am trying to 'recreate' this groupedData object. Here's what I've done:

x1=c(1,1,1,2,2,2,3,3,3,4,4,4,5,5,5,6,6,6);x2=c(55,53,54,26,37,32,78,91,85,92,100,96,49,51,50,80,85,83) eg1=data.frame(x1,x2);colnames(eg1)=c("Rail","travel");eg1gd=groupedData(travel~1|Rail,data=eg1) print(eg1gd)
x11();print(plot(eg1gd))
femodel=lm(x2~x1-1,data=eg1gd)
print(femodel$coefficients)

  Result:

        x1
  16.49817

...but, as you can see, the coefficients I get at the end this time are
completely different and I don't know why. Somehow, I am not creating the structure properly even though the formula and data values are all correct.

Can anyone help? I've looked at the ?groupedData man page, but it has no solution to this.

Thanks very much for any advice,
Toby

Pinheiro JC & Bates DM (2000). Mixed-Effects Models in S and S-PLUS (1st ed.). Springer, New York.



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