[R] Finney's fiducial confidence intervals of LD50

From: carlos riveira <carlos.riveira_at_yahoo.com>
Date: Mon 21 Aug 2006 - 22:03:50 EST

  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)
  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:
#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.    


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