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

[Hank Stevens]

HI Toby,
I think that the problem is that you have not specified using Rail and  
travel in the model statement, but rather are still using x1 and x2.  
It also does not realize that you want x1 to be a factor. Try

x1=factor(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)
femodel=lm(travel~Rail-1,data=eg1gd)
summary(femodel)

Cheers,
Hank
On Jun 4, 2008, at 10:01 AM, Toby Marthews wrote:

> 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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Dr. Hank Stevens, Associate Professor
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believe and adore." -Ralph Waldo Emerson, writer and philosopher  
(1803-1882)