Different Processor speeds with same steppings

Well I saw a bit of this mentioned in another thread but I thought I'd not go off topic and make a new thread.

So the question is when a new stepping is released with different stock speeds, how does AMD/Intel decide what speeds they use? I mean is there some quality/stability test for individual chips or to they just randomly throw chips in different baskets or what?

Apologies if this has already been asked somewhere.

Edit: Oops I can't fix the typo in subject