Hi Kathryn,
I've copied the following from the 'intro to phx" training manual; the exercise where we first introduce the Phoenix engine, I think there is a similar section comparing weighting in the user docs too -
Weighting in Phoenix Models
Phoenix WinNonlin Classic WinNonlin
Additive: C + CEps = Homoscedastic Uniform
Multiplicative: C*(1 + CEps) = Constant CV 1/Yhat*Yhat
Power: C + Cpower*Ceps (Yhat)-2*power
Power = 0.5: C + C0.5*CEps = Poisson 1/Yhat
Mixed: C + CEps + C*CEps*CMixRatio
Custom: user-defined
C is the predicted concentration, CEps is the residual error, CmixRatio is the ratio of proportional error to additive error.
Standard deviations (σ = Stdev(CEps)) are estimated instead of variances (σ 2 = Var(CEps)) for convenient interpretation
• Additive Stdev of 0.1 corresponds to the residual variability in the same units as the modeled concentrations (0.1 µg/L for example)
• Multiplicative Stdev of .10 corresponds to a 10% CV
• Mixed Stdev of 0.1 with CMixRatio of 1.5 means we have an additive component of 0.1 (µg/L, for example) and a proportional part of 0.1*1.5 = 0.15 = 15% CV
Simon.