How do you measure the productivity of a fireman?
Is it by the number of fires he put out?
By the number of cats rescued?
By the number of fires prevented?
Similarly, how do you measure the productivity of an artist?
By the number of paintings painted?
By the gallons of ink used?
By how much a collection of artwork sold for?
Similarly, how do you measure any unmeasurable creative field?
Perhaps, rather than trying to measure (and reward) slivers of productivity we just reward the process.