Inverse Document Frequency
		Consider a future device for individual use, which is a sort of mechanized private file and library. It needs a name, and, to coin one at random, "memex" will do. ¡ Words with higher ft are less discriminative. ¡ Use inverse to measure importance: l wt = 1/ft l wt = ln (1+ N/ft)  this one is most common l wt = ln (1 + fm/ft), where fm is the max observed frequency