Clergy work satisfaction

This study concludes that clergy are the happiest and most satisfied workers in the USA. I'm not sure how this tallies with statistics on clergy burnout, and with the number of ex-clergy now in other professions. Christian ministry can be the most difficult and frustrating of all callings... at the end of the day, how does one measure or evaluate one's work? At the same time, it can also be the most fulfilling of all callings.