NLMIXED in R

NLMIXED - Individual Variances

# Individual variancies for the standard samples
# weights = varIdent(form = ~ 1 | Standard.sample2)

fit8<-nlme(y~U+D*exp(-(Kd*x)),
                data=Grass_new, 
                fixed=list(U+D+Kd~Standard.sample2), 
                random=pdDiag(U+D~1),
                weights=varIdent(form=~1|Standard.sample2),
                method="REML",
                start=c(20,0,50.88,0,0.02698,0))
summary(fit8)
anova(fit7, fit8) # does not realy add anything --> no heteroscedastic model needed

plot(fit8, resid(.,type = "p")~ fitted(.)|Standard.sample2,id = 0.05, adj = -0.3 )
intervals(fit8)
plot(fit8, resid(.) ~ x |Trial,
     panel = function(x, y, ...) {
       panel.grid()
       panel.xyplot(x, y)
       panel.loess(x, y, lty = 2)
       panel.abline(0, 0)
     } )