Calculate an estimate of the F-measure based on the scores

def _calc_F_guess(self, alpha, predictions, probabilities):
        """Calculate an estimate of the F-measure based on the scores"""
        num = np.sum(predictions.T * probabilities, axis=1)
        den = np.sum((1 - alpha) * probabilities + \
                     alpha * predictions.T, axis=1)
        F_guess = num/den
        # Ensure guess is not undefined
        F_guess[den==0] = 0.5
        return F_guess