R help - Mar 2011 - adding a cubic spline to a model

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Greetings,

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I am trying
to add a cubic spline to an NLME biological growth model. After looking in the
R-help archive, I?ve tried the functions ns() and bs() from the splines-package
and rcs() from the Design-package:

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M7.W<-nlme(model= RW ~
(R*(RPrevYear/K)^m)*ns(Year,4)*exp(m*((1-RPrevYear^2/K^2)/2)), data=data,
fixed=R+K+m~1, random= R+K~1|IDPlot, start= c(R=3, K=90, m=0.1),
correlation=corARMA(p=2, q=0, form= ~Year))

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All of them
give the same error-message: 

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Error in
X[, fmap[[nm]]] <- gradnm : 

? number of items to replace is not a multiple
of replacement length

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Am I using
the ns() function wrongly or should I use another function?

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Thanks for
any hints,

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Ellen,

Msc
student, KULeuven, Belgium