[R] Fitting a model

Dani Valverde daniel.valverde at uab.cat
Thu Nov 20 18:48:10 CET 2008

Thank you all for your answers. If you look at the plot resulting from 
my data, it seems that it is some kind of sigmoid function, not only 
polynomial. How could I fit it?


Daniel Valverde Saubí

Grup de Biologia Molecular de Llevats
Facultat de Veterinària de la Universitat Autònoma de Barcelona
Edifici V, Campus UAB
08193 Cerdanyola del Vallès- SPAIN

Centro de Investigación Biomédica en Red
en Bioingeniería, Biomateriales y
Nanomedicina (CIBER-BBN)

Grup d'Aplicacions Biomèdiques de la RMN
Facultat de Biociències
Universitat Autònoma de Barcelona
Edifici Cs, Campus UAB
08193 Cerdanyola del Vallès- SPAIN
+34 93 5814126

En/na Eik Vettorazzi ha escrit:
> you might use the drc-package (equivalently you could use nls with an 
> appropriate "selfstart" model like SSlogis)
> library(drc)
> mm<-drm(delta~ph,fct=LL.4())
> plot(mm)
> From your plot I was assuming that "ph" is the independent variable 
> (as modelled above) - so if you want to predict a ph from delta you 
> will need the "inverse" function of your fitted model -  you could toy 
> with ED from the drc package or do a simple grid search with "predict".
> hth.
> Dani Valverde schrieb:
>> Hello,
>> This is a very basic question, but I don'y know the answer. I have 
>> these data
>> delta <- 
>> c(28.6-8.825,28.6-8.828,28.6-8.836,28.6-8.845,28.6-8.897,28.6-8.944,28.6-9.027,28.6-9.091,28.6-9.263,28.6-9.4,28.6-9.7,28.6-9.981, 
>> 28.6-10.287,28.6-10.48,28.6-10.684,28.6-10.875)
>> ph <- c(4.4,4.6,4.8,5,5.2,5.4,5.6,5.8,6,6.2,6.4,6.6,6.8,7,7.2,7.4)
>> plot(ph,delta,ylab=c(expression(Delta*delta)),xlab="pH")
>> Which kind of model can I fit on these, so that can I predict for a 
>> given delta the pH of my sample? Once the model is fitted, how can I 
>> plot it on the graph?
>> Best regards,
>> Dani

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