# How to translate Monolix Combined 1 ( additive + proportional) error model in Phoenix

**URL:** <https://forum.certara.com/t/how-to-translate-monolix-combined-1-additive-proportional-error-model-in-phoenix/4310>\
**Category:** General\
**Created:** [August 4, 2025, 8:22am UTC](https://forum.certara.com/t/how-to-translate-monolix-combined-1-additive-proportional-error-model-in-phoenix/4310 "2025-08-04T08:22:24Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Bhim\_Bahadur](https://avatars.discourse-cdn.com/v4/letter/b/f17d59/32.png) [@Bhim\_Bahadur](https://forum.certara.com/u/Bhim_Bahadur)\
**Post date:** [August 4, 2025, 8:22am UTC](https://forum.certara.com/t/how-to-translate-monolix-combined-1-additive-proportional-error-model-in-phoenix/4310/1 "2025-08-04T08:22:24Z")

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Hi, I am trying to replicate a published PopPK model built in Monolix with Combined1 error model:

 ![image](https://us1.discourse-cdn.com/certara/original/2X/2/2a414e6e7bfc7d31d2d1d29173357cb57a70313b.png)  
There we get additive and proportional error value. However, in Phoenix we have addi+multi error model as:  
observe(CObs = C + CEps \* sqrt(1 + C^2 \* (CMultStdev/sigma())^2)). So, how to define a and b in this?  
Currently, i understand is  
CEps = 1 and  
observe(CObs = C + CEps \* sqrt(a^2 + (b \* (C)^2)). Is this the right way to represent in Phoenix NLME, please guide.

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**Author:** ![SimonDavis](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@SimonDavis](https://forum.certara.com/u/SimonDavis)\
**Post date:** [August 6, 2025, 10:20am UTC](https://forum.certara.com/t/how-to-translate-monolix-combined-1-additive-proportional-error-model-in-phoenix/4310/2 "2025-08-06T10:20:39Z")

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Hi Bhim, what I would suggest is use the built-in or graphical models to learn some of the syntax conventions; as you change the options you will see the generated code and statements change for you which is a great way to learn e.g.  
(Model text and also Warnings tab can be informative too !)

 ![image](https://us1.discourse-cdn.com/certara/original/2X/8/8dbb1867735399cfab43ad58e247937192aa1ec9.png)

so typically we represent as you said as  
**observe(CObs = C + CEps \* sqrt(1 + C^2 \* (CMultStdev/sigma())^2))**  
I would recommend to look at the PML guide, particularly page 21 found under [https://help.certara.net/en/articles/10769710-phoenix-8-6-x-documentation](https://help.certara.net/en/articles/10769710-phoenix-8-6-x-documentation) which explains that

Only a single error variable can be used in an observe statement such as the one on line 23.  
Compound residual error models for any given observed variable, such as mixed additive  
and proportional, must be built using a combination of fixed effects and a single error vari-  
able rather than multiple error variables. For example, the following statements are correct:  
**stparm(CMixRatio = tvCMixRatio)**  
**fixef(tvCMixRatio = c(, 0.1, ))**  
**error(eps1 = 0.01)**  
_observe(cObs = c + eps1(1 + c\*MixRatio))_\*  
The following statements are incorrect as the observe statement contains two error vari-  
ables, eps1 and eps2:  
error(eps1 = 0.01)  
error(eps2 = 0.001)  
observe(cObs = c\*(1 + eps2) + eps1)

For more information on the observe statement, see the “ **Observations** ” section.

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**Author:** ![Bhim\_Bahadur](https://avatars.discourse-cdn.com/v4/letter/b/f17d59/32.png) [@Bhim\_Bahadur](https://forum.certara.com/u/Bhim_Bahadur)\
**Post date:** [August 23, 2025, 3:55am UTC](https://forum.certara.com/t/how-to-translate-monolix-combined-1-additive-proportional-error-model-in-phoenix/4310/3 "2025-08-23T03:55:18Z")

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Thank you **[SimonDavis](https://forum.certara.com/u/simondavis).**