I'm trying to figure out what exactly defines the CR of a creature. Without the dozen playtests with different levels of groups, is there a quick easy way to do this?
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Yes and No. The original 3.5E rules include methods of generating CR from improving existing monsters (i.e. templates, which have CR adjustments) or creating a brand new monster (basing CR off of Hit Dice and other factors.) However, these values, like challenging ratings in general, are only approximations, and should never be considered as "exact numbers." CRs are guidelines, nothing more and nothing less, and should be taken with grain of salt both when using them and when creating them. The difficulty of an encounter is very much variable depending on all sorts of situations, including the environment and the specific PCs facing off against it. Playtesting over the course of many sessions or campaigns, as with all game design, is the best method for achieving a good balance.
A CR starts with a baseline of the number of HD, assuming appropriate HD by type, appropriate saves by type, and no non-type-standard specials.
It's adjusted up for bigger than normal-for-type HD, not-standard-for-type special abilities, improved saves, high stats.
It's adjusted down for smaller than normal-for-type HD, special weaknesses, lowered saves, lowered BAB, or low stats.
They're then adjusted based upon playtester feedback. Some combinations of powers boost more than would be expected; others preclude each other's simultaneous use, and thus add less than might be expected.
Pathfinder uses the same base assumptions as D&D 3.X, but the specifics are just enough different that some CR variations occur.