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
Tel:86-21-54237149

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