# Inverse prediction from binomial glm - how to obtain confidence intervals?

I am trying to use a glm with a binomial distribution (logit link) to analyse data from a dose response curve for the lethal effects of different bacterial strains. I now want to obtain estimates of the lethal dose 50 (LD50).

My model is simply of the form:

survival ~ a + b*dose


Thus since I have a logit link, obtaining the estimates for the LD50 (i.e. p=0.5) simply means dividing -a/b. However, I am also wanting to know the confidence intervals for these estimates and that's where I am unsure about what to do. If anyone would have a suggestion for this, that would be great.

• Depending on your sample size, you could use bootstrapping to obtain CIs. – boscovich Nov 30 '12 at 19:10
• Hi Andrea, Thanks for your reply. I have 5 datapoints per dose (4 doses). Could you please further explain how the bootstrapping could be done in this case? (sorry, I haven't really used this much). Cheers, Ricardo – ricardo Dec 1 '12 at 14:37
• Have a look at this link. – Hans Roggeman Feb 2 '14 at 3:03