Thanks Rick, using it now. Note that the text has Δt = log(81)r, but the code is Δt = log(81)/r. I don't know what the confidence intervals shown mean though, as the data is not iid (independently and identically distributed)? As compared to if the data at each timepoint was always from a different set of 58 plants. In other words, if you used NLMIXED on the original data with correlated data over time from 58 experimental units, would you get the same plot and narrow CI as here? My data is from 24 animals and x is drug dose per kg. The dose response curve is s-shaped.