I am having some confusion on how CR scales.

[Macgreine]

Lets say I have a Shambler that is CR6 and I add another shambler which is CR 6 and two Bugbears which are CR2 each, what is the CR of the encounter and how do you figure it out?
Thanks for help, Mac

[Vod Canockers]

Add all the CRs together, and compare the total to the chart.

[Ximen Bao]

I thought you added the XP together and compared it to the chart here:
http://paizo.com/pathfinderRPG/prd/gamemastering.html#_designing-encounters

[Mama Loufing]

Add the XP for each together and then compare that total to the Experience Points Awards Table.

2 Shambling Mounds @ 2400 XP = 4800
2 Bugbears @ 600 XP = 1200
Total 6000 = CR 8

At 6400 it becomes CR 9.

To make it all easier, try Kyle Olsen's Combat Manager. You can add the monsters to the combat and it calculates the CR automatically.

[Macgreine]

Thanks. That makes total sense. I appreciate your quick replies. :)