Well, there is the wet kind, the dry kind, each could be loud or silent with probability correlated to the moisture content, then you have the event length, which is correlated to the exhaust gas velocity and total volume expelled. Of course, depending on the health of the individual at the time, you might need to factor in the possible presence of chunks, etc. I think Dave could work with that challenge.
Cross all that with other key organoleptic characteristics (e.g., smell) and you have the basis for a classification system. I do find it funny that the insertion of fact information in an opinion-information thread gets labeled clusterfart. (Acree: you need a semicolon not a comma there after precious. )