Some of the problem is scale: The bigger the community the more idiots you’re bound to have. There is some ambient level of morons that you just can’t get rid of. If you think of idiots as a contaminant, then it may be useful measure them as a portion of the whole. Say: Idiots Per Thousand. The IPT of any given site can’t go below a certain base value: The level of background idiocy on the internet. But idiots have a tendency to drive normal people away if they are allowed to run unchecked, thus increasing the saturation of IPT.
There is a more elegant name for this phenomenon, but i can’t remember what it is. (Shamus Young)