# [R] Info

Spencer Graves 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.)

1.  Plot the data.

2.  Play with the nonlinear formula to understand what the particular
coefficients do.

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.

Best Wishes,
Spencer Graves

Andrea Calandra wrote:
> Sorry
>
> 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])
>
> where
> 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
> Andrea
>
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