Perhaps you really prefer something with a continuous first derivative? In that case, all the continuous cumulative distribution functions have a sigmoidal shape and might be suitable. You could fit pnorm, plogis or tanh with suitable scaling and location parameters using nls. An example of fitting a logistic is at
https://stat.ethz.ch/pipermail/r-help/2004-April/048385.html TAPO (Thomas Agersten Poulsen <tapo <at> novozymes.com> writes: : : Dear R-list, : : It is not uncommon for laboratory equipment (e.g. spectrophotometers) to have a linear response in a : certain interval and then go into saturation. I wonder if there is an R- function that models this; for : instance by estimating the breakpoint and fitting a line below the breakpoint and a constant above. : : Best regards : Thomas Poulsen ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html