[R] Problem with 'nls' fitting logistic model (5PL)

[Michal Figurski]

Dear R-Helpers,

I'm working with immunoassay data and 5PL logistic model. I wanted to 
experiment with different forms of weighting and parameter selection, 
which is not possible in instrument software, so I turned to R.

I am using R 2.14.2 under Win7 64bit, and the 'nls' library to fit the 
model - I started with the same model and weighting type (1/y) as in the 
instrument to see if I'll get similar results. However, in some 
instances I don't get any results - just errors.

Here is an example calibration data, representative of my experiment. 
Instrument soft had no problem fitting it:
x <- structure(list(SPL = structure(c(1L, 1L, 2L, 2L, 3L, 3L, 4L,
4L, 5L, 5L, 6L, 6L, 7L, 7L), .Label = c("St1", "St2", "St3",
"St4", "St5", "St6", "St7"), class = "factor"), MFI = c(10755.5,
9839, 5142.5, 4857, 1510.5, 1505, 502.5, 451, 215, 195.5, 58,
57, 15, 15), nom = c(206, 206, 125, 125, 68, 68, 38, 38, 24,
24, 13, 13, 6.5, 6.5), weights = c(0.0013946353028683, 0.00152454517735542,
0.00291686922702965, 0.00308832612723904, 0.0099304865938431,
0.00996677740863787, 0.0298507462686567, 0.0332594235033259,
0.0697674418604651, 0.0767263427109974, 0.258620689655172, 
0.263157894736842,
1, 1)), .Names = c("SPL", "MFI", "nom", "weights"), row.names = c(NA,
-14L), class = "data.frame")

And here is the nls fit:
fit <- nls(MFI~a + b/((1+(nom/c)^d)^f), data=x, weights=x$weights, 
start=c(a=100, b=10000, c=100, d=-1, f=1))

I've tried every possible combination of starting values, including the 
values fitted by the instrument soft - to no avail. I've probably seen 
all possible error messages from 'nls' trying to fit this.

If anyone has an idea why it's not working - let me know.

Best regards,

-- 
Michal J. Figurski, PhD
HUP, Pathology & Laboratory Medicine
Biomarker Research Laboratory
3400 Spruce St. 7 Maloney S
Philadelphia, PA 19104
tel. (215) 662-3413