Dear Irene, if you have a vector of estimates of x1, x2, y1, y2 and the corresponding estimated variance-covariance matrix (accessible through the method "vcov"), then one possibility is to use the function delta.method() in the package alr3 (on CRAN).
This function returns: 1) an estimate of the ratio (which is simply obtained by plugging in the estimates of x1, x2, y1, y2) and 2) an approximate, large-sample standard error of the estimated ratio Christian ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
