def __getVariance(self):
        """Determine the measure of the spread of the data set about the mean.
        Sample variance is determined by default; population variance can be
        determined by setting population attribute to True.
        """

        x = 0	# Summation variable.

        # Subtract the mean from each data item and square the difference.
        # Sum all the squared deviations.
        for item in self.sample:
            x += (item - self.mean)**2.0

        try:
            if (not self.population):
                # Divide sum of squares by N-1 (sample variance).
                self.variance = x/(self.N-1)
            else:
                # Divide sum of squares by N (population variance).
                self.variance = x/self.N
        except:
            self.variance = 0