Message-ID: <008001c31498$dc751130$0201a8c0@MARC>
Date: 2003-05-07T13:02:04Z
From: Marc Schwartz
Subject: [R ] Query : problems with the arithmetic operator "^" withfunction "lme" and "lmList"
In-Reply-To: <200305071257.h47CvcwR023819@tom.ens.univ-reims.fr>
>-----Original Message-----
>From: r-help-bounces at stat.math.ethz.ch
>[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of MARTIN
Ludovic
>Sent: Wednesday, May 07, 2003 7:02 AM
>To: r-help at stat.math.ethz.ch
>Subject: [R] [R ] Query : problems with the arithmetic
>operator "^" withfunction "lme" and "lmList"
>
>
>Dear all,
>
>I've got a problem in including square variables in lme and lmlist
>functions. I've tried to work on Oxboys data of Pinheiro and
>Bates'book, which consist of the heights of 26 boys, each mesured on
>nine different occasions :
>
> > Oxboys
>Grouped Data: height ~ age | Subject
> Subject age height Occasion
>1 1 -1.0000 140.50 1
>2 1 -0.7479 143.40 2
>3 1 -0.4630 144.80 3
>4 1 -0.1643 147.10 4
>5 1 -0.0027 147.70 5
>...
>
>A quadratic model in "age" would be fit with lmList
>(height~age+age^2,Oxboys).
>
>We obtain:
>
>> fm2Oxboys.lis<-lmList(height~age+age^2,Oxboys)
>> fm2Oxboys.lis
>Call:
> Model: height ~ age + age^2 | Subject
> Data: Oxboys
>
>Coefficients:
> (Intercept) age
>10 130.2616 3.722915
>26 137.9927 5.588783
>25 139.2105 4.024081
>9 138.1369 6.009057
>2 142.8584 5.440176
>...
>19 164.5761 9.065620
>4 165.0724 9.360561
>
>Degrees of freedom: 234 total; 182 residual
>Residual standard error: 0.6598878
>
>They are not coefficients associated with the covariate "age^2" !
>However, the model called is " Height~age+age^2 | Subject " .
>
>I would be grateful if anyone could help me.
>
>Cordially,
>
>Martin Ludovic.
Respecify your model formula as:
fm2Oxboys.lis<-lmList(height ~ age + I(age^2),Oxboys)
Note the use of I() around the "age^2", which enables R to interpret
the transformation of age squared properly.
See:
?formula and ?I
Hope that helps.
Marc Schwartz