1 reply to this topic

[Kathryn]

Hi

What is the weighting used in the phoenix model? I see the column 'Weight' on the Residuals tab of the output, but I don't see how to change the weighting in the simulation settings. For example I want my weighting to be 1/standard deviation

Kathryn

[Simon Davis]

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.