RE: interpretation of WRES of logarithmically transformed data

From: VPIOTROV@PRDBE.jnj.com Subject: RE: [NMusers] interpretation of WRES of logarithmically transformed data Date: 11/24/2003 10:26 AM The only need to set IPRED to an arbitrary value (-3 in Anthe's case) is to protect from the run stop due to log domain error since the model may accidentally return F<=0. Certainly F = 0 during the lag time. If the IPRED value selected for those cases deviates substantially from the range of DV for your data, this will serve as a flag helping to locate subjects with "problematic" measurements. Of course actions should be taken to avoid situations when Tlag >= time of the 1st sample with a measurable concentration. In the simulation study that Ken cited, Stuart suggested the following residual error model, which seems to solve the problem of nonzero measurements during the lag-time period (i.e., when the model predicts zero concentration): log(y(t)) = log(f(t)+m) + f(t)*e1(t)/(f(t)+m) + m*e2(t)/(f(t)+m) I wonder if somebody from the list tested this model. I did not test it myself yet as with the absorption model I use routinely long Tlag is almost always avoided. Best regards, Vladimir