Hallo, in order to compare three preparations of a quantal bioassy I perform a probit analysis. To obtain Fieller's confidence limits for the relative potency I need the covariance parameters of the estimated coefficients. I used two methods,'glm' and 'glim' in S-PLUS 4.5 for Windows, but got extremley different covariance matrices, while only moderate differences in the coefficients occur.
Differences seem to be due to a common mulitplicity constant for all covariance parameters.
I also recognized that covariance estimates differ by changing the 'wt' or 'weights' arguments.