'The observed disagreement for the alpha coefficient.
The alpha coefficient, unlike the other metrics, uses this rather than
observed agreement.'

def Do_alpha(self):

    total = 0.0
    for (i, itemdata) in self._grouped_data(u'item'):
        label_freqs = FreqDist((x[u'labels'] for x in itemdata))
        for (j, nj) in iteritems(label_freqs):
            for (l, nl) in iteritems(label_freqs):
                total += (float((nj * nl)) * self.distance(l, j))
    ret = ((1.0 / ((len(self.I) * len(self.C)) * (len(self.C) - 1))) * total)
    log.debug(u'Observed    disagreement:    %f', ret)
    return ret
