[R] do i set the correct argument?

From: zhijie zhang <epistat_at_gmail.com>
Date: Tue 04 Jul 2006 - 00:15:50 EST

Dear friends,
 In gls() of nlme package, there is some explanation on correlation: gls(model, data, correlation, weights, subset, method, na.action, control, verbose)

 correlation: an optional 'corStruct' object describing the within-group

          correlation structure. See the documentation of 'corClasses'
          for a description of the available 'corStruct' classes. If a
          grouping variable is to be used, it must be specified in the

'form' argument to the 'corStruct' constructor. Defaults to
'NULL', corresponding to uncorrelated errors.
 Now i have two categorical variables: grass and altitude, *gls.fit
<-gls(log(snail)~grass+altitude+gheight+humidity+soiltemr+airtemr,data=model,correlation=corAR1( form=~grass+altitude))*
 Is the above argument that i use right? thanks very much!
Kind Regards,
Zhi Jie,Zhang ,PHD
Department of Epidemiology
School of Public Health
Fudan University

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