Returns the f-measure, the combination of precision and recall based on
parameter beta - default = .5 which => precision and recall are equally weighted.
beta = 1 => precision is maximized. beta = 0 => recall is maximized.

From a recent statistics book - All of Statistics - springer verlag
http://www2.springeronline.com/sgw/cda/frontpage/0,,4-10128-22-13887455-0,00.html