3.5 Challenge Ratings

[Alysar]

I have a question. Is an appropriate challenge rating for a party the average of all the characters' levels, or is it the sum of the characters' levels divided by four?

I mean, are the challenge ratings in the books calculated to assume a party of four members?

And what if one or more of the characters has a cohort?

[Dairun Cates]

Honestly, the CR system is kinda borked to begin with. Just run around about what seems right for your party's skill level after a few combats. Divide experience for the actual party.

[Zeta Kai]

I have a question. Is an appropriate challenge rating for a party the average of all the characters' levels, or is it the sum of the characters' levels divided by four?

It's the average of all party member's ECLs. In a party of 4 PCs, the two numbers are identical.

I mean, are the challenge ratings in the books calculated to assume a party of four members?

Yes, inasmuch as they are calculated at all, a party of 4 PCs is the presumed default.

And what if one or more of the characters has a cohort?

IIRC, cohorts don't counts toward the ECL of a party for determining a challenging encounter.

Also, this is NOT homebrew.