# CObs question **URL:** \ **Category:** Modelling and Simulation\ **Created:** [May 22, 2011, 1:51pm UTC](https://forum.certara.com/t/cobs-question/2293 "2011-05-22T13:51:30Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![dora\_teo](https://avatars.discourse-cdn.com/v4/letter/d/7ea924/32.png) [@dora\_teo](https://forum.certara.com/u/dora_teo)\ **Post date:** [May 22, 2011, 1:51pm UTC](https://forum.certara.com/t/cobs-question/2293/1 "2011-05-22T13:51:30Z")
Hello, I need some help with a more general modeling problem: suppose that I have observations from two matrices (plasma and blood) collected at the same time points. How can I set up my model to simultaneously describe the concentrations in both matrices when one matrix is contained in the other? I tried to set it up using a 2 or 3 compartment model in which I map plasma data to the central compartment (CObs1= Cplasma + CEps1) and then using the model text editor write: (CObs2 = Cplasma + Cperipheral + CEps2). I am pretty new to modeling and to phoenix environment and I am wondering if this is a correct way of doing it. Any suggestions would help. Thanks! Teodora ---
**Author:** ![SimonDavis](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@SimonDavis](https://forum.certara.com/u/SimonDavis)\ **Post date:** [May 24, 2011, 4:36am UTC](https://forum.certara.com/t/cobs-question/2293/2 "2011-05-24T04:36:20Z")
One suggestion as to how this could be written: C2 = CPlasma + CPeripheral # separate statement defining C2 observe(CObs2 = C2 + CEps2) # observe CObs2, with C2 as the prediction This gives the prediction a name, C2. (It doesn’t have to be C2. It can be any new name that’s not already in the model.) Simon