Grokbase Groups R r-help January 2004
FAQ
Hi:

I have data from an assay in the form of two vectors, one is response
and the other is a predictor. When I attempt to fit a 5 parameter
logistic model with "nls", I get converged parameter estimates. I also
get the same answers with "gnls" without specifying the "weights"
argument.

However, when I attempt to use the "gnls" function and try to estimate
the variance function, as a power function, I get the following error
message:
ans51g <- gnls(log(b51) ~ p0 + p1/(1 + exp(-(log(dose)-p2)/p3))^p4,
start=list(p0=3,p1=1,p2=4,p3=2,p4=1.5),control=gnlsControl(tol=1.e-
07),weights=varPower())
Error in eval(expr, envir, enclos) : Object "." not found
>

What am I doing wrong here and how can I do a GLS analysis with a
variance function that is estimated from the data?

Here is my data:
b51 <- c(17447.60674, 7060.37234, 2872.53012, 796.40426,
454.47222, 260.22340, 120.11905, 83.40196, 51.45745,
36.87912, 26.73256, 25.18681, 17.97674)
dose <- c( 1.000000e+04, 1.000000e+03, 2.500000e+02, 6.250000e+01,
3.125000e+01, 1.562500e+01, 7.812500e+00, 3.906250e+00,
1.953125e+00, 9.765625e-01, 4.882813e-01, 2.441406e-01, 1.000000e-03)

thanks for the help,
Ravi.

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