'Return the overall F measure for all texts that have been
scored by this ``ChunkScore``.
:param alpha: the relative weighting of precision and recall.
Larger alpha biases the score towards the precision value,
while smaller alpha biases the score towards the recall
value.  ``alpha`` should have a value in the range [0,1].
:type alpha: float
:rtype: float'

def f_measure(self, alpha=0.5):

    self._updateMeasures()
    p = self.precision()
    r = self.recall()
    if ((p == 0) or (r == 0)):
        return 0
    return (1 / ((alpha / p) + ((1 - alpha) / r)))
