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.