Dear all,
I'm trying reproduce an example of bioassays Yielding Concentration-Mortality Data particularly control - adjustment model from book Bioassay of Entomopathogenic Microbes and Nematodes chapter 7 with R.

I used glm with family=binomial and link=probit, but I do not know how to implement parameter gamma (control mortality - mortality of the untreated control insect in this exaple) into model formula.

Model in book:
pi(x)=gamma+(1-gamma)F(alpha+beta log(x))

F....cumulative probability distribution function 
data:
x=concentration
n=number of insect at each run
y=number of death among n in given batch run at given concentration 


Xmat<-data.frame(x=rep(c(0.01,0.1,1,10,100),2),n=rep(100,10),y=c(19,20,21,45,80,25,25,27,56,91))



Reults from book (obtained from SAS) are

intercept  -1.6597 , beta =0.5586, control mortality --- gamma =0.2172

Thanks for any advice.
Michael

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