[R] Inverse of Probit

Calum calummclean at hotmail.com
Tue Nov 19 19:59:01 CET 2013

Hi there,
I hope someone can help me.

I have a dataset of Concentration against Mortality, and I am trying to
compare the use of Logit and Probit models using this data.

The issue I am having is trying to back transform the data from the probit
model, to plot it in normal space instead of log space.
I know this can be done with a logit model using the code below, where
ilogit is a function for the inverse logit:

NEWCONC <- seq(0,0.6, length=25)
NEWMORT <- predict(LOGIT, Conc=NEWCONC, se=TRUE)

plot(data=DATA, Prop~Conc)
lines(NEWCONC, ilogit(NEWMORT$fit))

However, I can't seem to find a function equivalent to ilogit for a probit
model, that I could use in this code:

NEWCONC <- seq(0,0.6, length=25)

plot(data=DATA, Prop~Conc)

Any advice on this issue would be appreciated,

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