'Scott 1955; here, multi-pi.
Equivalent to K from Siegel and Castellan (1988).'

def pi(self):

    total = 0.0
    label_freqs = FreqDist((x[u'labels'] for x in self.data))
    for (k, f) in iteritems(label_freqs):
        total += (f ** 2)
    Ae = (total / ((len(self.I) * len(self.C)) ** 2))
    return ((self.avg_Ao() - Ae) / (1 - Ae))
