The scale is divided into 10 categories... with 1 being the lowest, and 10 being the highest.
1: Slight perp... sometimes silent. Usually detected within 30 seconds by a well trained nose. Best released by lifting one half of the arse and slowly allowing gas discharge. Usually has the smell of a warm biscuit.
2: Gas Shot... Can be heard if not carefully released, detection time is same as above. Very little Vibration if arse is firmly planted on chair / bench / sofa. Usually let off in queues, usually in Banks or where people are buying a Lottery ticket.
3: Flustered Fart... Can be heard in quieter surroundings, usually let off when walking. Never detected by its owner, but by the next poor soul who stops nearby the release site. Best dropped when walking away from a cash machine.
4: Parppp... Definitely detectable by anyone in close proximity. Higher volume output than that of the afforementioned scale factors... Can be used to show off to mates, or to be kicked out of bed by the missus. Slight vibration occurs if directly aimed onto a spring based matress.
5: Trump... Most commonly heard in working mens clubs, and changing rooms. Men find a sense in pride in dropping these, although women (other than wives married over 5 years) find this type of fart impossible to acheive in the presence of any...