[R] info

Douglas Bates bates at stat.wisc.edu
Fri Jul 11 17:42:43 CEST 2003

Andrea Calandra <a.CALANDRA at mclink.it> writes:

> I'm a student in chemical engineering, and i have to implement an algoritm about FIVE PARAMETERS INTERPOLATION for a calibration curve (dose, optical density)
> 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
> Have you never seen this formula, because i don't fine information or
> lecterature about solution of this!!!

This is one parameterization of the four-parameter logistic growth
curve.  A slightly different version is available as the selfStart
model SSfpl in the nls package.  In R try


to see how nls and SSfpl can be used.  The example even produces a
figure for you showing what the SSfpl parameters represent.

A literature reference for the SSfpl form of the four-parameter
logistic is Appendix C.6 in Pinheiro and Bates (2000), "Mixed-effects
Models in S and S-PLUS", Springer.

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