RE: simulated negative concentration

From:"Kowalski, Ken" Subject: RE: [NMusers] simulated negative concentration Date:Fri, 14 Feb 2003 08:13:52 -0500 Yaning, You must have a large SD for the additive component relative to F at low concentrations and/or a large CV for the proportional component. Note that truncating the concentrations with this error model does result in bias although it may be negligible (depending on the severity of the trucation that is required to avoid negative concentrations). You might consider refitting your data using a log-transform-both-sides approach which would restrict simulated concentrations to be positive. To do this you need to define the logarithm of the concentrations as your DV and use a statement like Y=LOG(F)+EPS(1). Of course you need to avoid situations where the model can predict F=0. Ken