[R] probit analysis

[arun]

Hi Trinh,

Please check ?dose.p() from library MASS.

Since you wanted the rate, the code was a bit modified from the example.  

> datPr
       X Event Trial
1 1210.0     8     8
2  121.0     6     8
3   60.5     6     8
 fm<-glm(Event/Trial~X,data=datPr,family=quasibinomial(link=probit))
 XD50<-dose.p(fm,cf=1:2,p=0.5)
              Dose       SE
p = 0.5: -203.7126 105.3647

 XD50_CI<-XD50+c(1,-1.96,1.96)*attr(XD50,"SE")
 names(XD50_CI)<-c("XD50","lower","upper")
 XD50_CI
       XD50       lower       upper 
 -98.347938 -410.227309    2.802128 

I am not very confident about the results though.
 

 A.K.
 

From: phale_chuong gionho <phale_chuonggionho at yahoo.com>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc: 
Sent: Monday, June 4, 2012 10:52 AM
Subject: [R] probit analysis

 Hello!
> I have a very simple set of data and I would like to analyze
> them with probit analysis.
> The data are: X    Event    Trial
> 1210  8        8
> 121  6        8
> 60.5  6        8
> I want to estimate the value of X that will give a 95% hit
> rate (Event/Trial) and the corresponding 95% CI.
> you can help me? Thanks!!
> Trinh 

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