proportional error parameter (tvCMultStd) as a negative value?

Phoenix Certara Forums Modelling and Simulation
1

Hi

Is it possible to estimate the proportional error parameter (tvCMultStd) as a negative value?
I am trying to build a population Pk model using a combined additive and proportional error model; however, I got tvCMultStd as a negative value. Interestingly, when I use only a proportional error model, it is estimated as a positive value. I want to use a combined additive and proportional error since it shows a much better fit confirmed by AIC and diagnostic plots than the proportional error model. Still, I have a concern regarding the negative estimation of tvCMultStd.

Can I get any explanation regarding this?

Thanks

2

Dear David,

You are correct and tvCMultdStd should not be negative. However, in Phoenix, tvMultStdev is treated as a regular fixed effect and thus the reason it could become negative. In training, we always recommend that a lower bound of 0 is added to this parameter.

Hope this helps,

Ana Henry

3

Dear Ana

Thanks for your help. Your suggestion has fixed the issue!