Hi Greg, you can use the extension package 'drc' from CRAN:
conc <- c(10.3, 10.8, 11.6, 13.2, 15.8, 20.1) # Exposure concentrations orign <- c(76, 79, 77, 76, 78, 77) # Original number of subjects ndead <- c(16, 22, 40, 69, 78, 77) # Number dead after 96 h d <- data.frame(conc=conc, orign=orign, ndead=ndead) ## Loading 'drc' library(drc) ## Fitting model assuming 100% mortality for high concentrations ## and 0% for concentration 0 d.m1<-drm(ndead/orign~conc, weight=orign, data=d, fct=W1.2(), type="binomial") plot(d.m1) ## Fitting model where mortality at conc=0 is estimated ## (ad hoc adjustments should be avoided) d.m2<-update(d.m1, fct=W1.3u()) plot(d.m2, add=TRUE, type="none", lty=2) ## Calculating LC50 ED(d.m2, 50) Note that you don't really have time-to-event data as you only have observations reflecting the effect of exposure at one particular time point (96h). Time-to-event data occur if exposure is monitored repeatedly over time, in several intervals or at several time points, e.g. at 24h, 48h, and 96h, possibly subject to censoring. Therefore the above analysis is based on binomial distributions that are often appropriate for quantal data. Note also, that in ecotoxicology "Weibull model" may refer to one several related models depending on which guidelines, papers, or software you refer to. 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.
