I am trying to calculate a power curve (Freundlich equation) in R using drm based on, https://www.statforbiology.com/nonlinearregression/usefulequations. Here is the code:
model <- drm(numSpecies ~ Area, fct = DRC.powerCurve(),
data = speciesArea)
DRC.powerCurve() provides an estimate for the first derivative (i.e., slope), however, I was wondering if it's possible to get the second derivative? According to R documentation (http://ftp.uni-bayreuth.de/math/statlib/R/CRAN/doc/packages/drc.pdf) you can specify which parameters to estimate in the 'fixed' statement
LL.4(fixed = c(NA, NA, NA, NA), names = c("b", "c", "d", "e"), ...)
However, when I try to do this in R with DRC.PowerCurve setting the parameter estimates to zero, I receive the error, "Not correct 'fixed' argument". I'm also unsure which initial correspond to the second derivative.
I can find little to no information on DRC.PowerCurve, let alone obtaining the second derivative. In fact, DRC.PowerCurve isn't even listed as a potential function in R documentation, or in getMeanFunction().
