Standard Error in ka with Decreasing Data

It's probably not NONMEM, but the model misspecification associated with use of a first-order absorption model. If, say, everyone had their tmax at the same time, and everyone had the same elimination half-time, then ka would be perfectly determined from data only at tmax and later. Now if you add some weird concentrations earlier than tmax, they don't fit, and cause the estimate of ka to be less secure .. that's an idea, anyway ... of course one wants to look at the whole fit. There should be graphical evidence that the absorption model does not fit the early points for my explanationto be valid ...