I can't remember if there is anything about it in Doreen's fabulous Sweeteners article BUT .... since my machine is so dang slow, I'll just add a wee bit to this subject:

Stevia is actually extracted and processed from the leaf of the Stevia plant not the root, and it's "available" in 3 forms, green leaf (strong aftertaste though), white purified powder extract and ready-to-use liquid (which is the powder in water or glycerine or alcohol). I make my own liquid from the white powder, most economical and controls the strength. I use 1 Tsp white powder to 3 Tbsp filtered water and put it in a little dropper bottle (found great ones at the dollar store); 1 Tsp of this liquid is *approximately* equal to 1 Cup of sugar in sweetness, though this can vary with the batch of powder and should ALWAYS be *under* measured in recipes until sure. Too much stevia, like any sweetener, can produce a dreadful aftertaste...

Enjoy!