  I am working with Probit regression (I cannot switch to logit) can anybody help me in finding out how to obtain with R Finney's fiducial confidence intervals for the levels of the predictor (Dose) needed to produce a proportion of 50% of responses(LD50, ED50 etc.)? 
  If the Pearson chi-square goodness-of-fit test is significant (by default), a heterogeneity factor should be used to calculate the limits.
   
  Response<-c(0,7,26,27,0,5,13,29,0,4,11,25)
  Tot<-rep(30.5,12)
  Dose<-rep(c(10,40,160,640),3)
  probit<-glm(formula = Response/Tot~ log10(Dose), family=quasibinomial
  (link=probit))
  D50<- round(10^(dose.p(probit,cf=1:2,p=0.5)))
   
  #This is what SPSS calculates. I would like to reproduce these results with R:
  #SPSS RESULTS:
  #PRNT50= 140,83525
  #CI = 98,37857;205,34483
  #Regr.coeff= 1,91676 (S.E.=0,16765)
  #Intercept=-4,11856 (S.E.=0,36355) 
  Thanks a lot for your help.
   
  Carlos

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