spencer.graves at pdf.com
Thu Jul 17 17:02:00 CEST 2003
Thanks for providing data and sample command. (In the future, please
provide the data in the form of a "data.frame" commmand. It is not as
easy to read visually, but it is easier for others to copy into R.)
Please follow Doug Bates' advice:
1. Plot the data.
2. Play with the nonlinear formula to understand what the particular
3. Use algorithm = "plinear".
** If you did the above, you would find out that you "data" are
entirely linear. What values for the parameters to you need to make
your nonlinear formula (approximately) linear? I don't have time to
analyze it right now, but you may need to send b to 0 and c. to Inf.
Compute a second-order Taylor approximation to your formula to find out.
Also, please set "trace=TRUE": Then the algorithm will tell you what
it is trying to do with the parameter estimates.
You are persistent, Andrea: You will get an answer.
Andrea Calandra wrote:
> I'm student in biomedical engineer and i have to solve this formula
> for immuno-assay. I need to design a calibration curve
> But i don't understand How can i write this formula in R language:
> y = a + (c - a) /(1+ e[-b(x-m])
> x = ln(analyte dose + 1)
> y = the optical absorbance data
> a = the curves top asymptote
> b = the slope of the curve
> c = the curves bottom asymptote
> m = the curve X intercept
> I have to calculate the parameters (a,b,c,m).After with X that i know
> i calculate the Y.
> i try:
> yeld.fit <- nls( y ~ a + (c.-a)/(1+exp(-b*(x-m))),
> data = yeld,
> start = list( a= 0, c.=2, b= 1, m=4 ),
> trace = TRUE )
> where yeld is a data.frame
> x y
> 1 1 1
> 2 2 2
> 3 3 3
> 4 4 4
> 5 5 5
> but give me an error: << exceeded number of itwerations>>
> thank you
> R-help at stat.math.ethz.ch mailing list
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