nlme and start=list()

Andrew Beckerman <a.p.beckerman at stir.ac.uk> writes:

Dear Listers -

R1.61 on NT

We are trying to fit a polynomial usning nlme so that in ax^2+bx+c, c
is fixed and a and b are random.


The data are
Frames = speed of flight (20-50)
Manlength = tail length (-20,0)
ID = individual (14 of them)

each individual has experienced each of 6 tail lengths in the range
above and we are interested in the shape of the relationship between
speed and manipulation.


using the following approach based on the Venables and Ripley (3rd ed)
examples with the Sitka data set


nlme(Frames~a*Manlength^2+b*Manlength+c,
+ fixed=list(c~1),random=a+b~1|ID,
+ start=list(fixed=c(0.3,6,50)))

Error in nlme.formula(Frames ~ a * Manlength^2 + b * Manlength + c,
fixed = list(c ~  :

         starting values for the fixed component are not the correct length

-realizing that something is wrong with how we have framed the start
values... can anyone advise?

First the good news - you don't need to use nlme to fit a polynomial
model.  A polynomial is linear in the coefficients so you can use lme,
which is much easier than nlme.

lme(Frames ~ Manlength + I(Manlength^2), random = Manlength)

The problem with your usage of nlme is that you gave 3 values for the
starting estimates of the fixed-effects parameters and there is only
one parameter for fixed-effects the way you wrote it.  You should have
used "fixed=list(a+b+c ~ 1)" (or, equivalently, "fixed=a+b+c~1") but
you really don't want to use nlme for a linear model.
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