/* The std dev of the mean is related to the std dev by:  */
static double stddev_stats(struct stats *stats)
/* The std dev of the mean is related to the std dev by:  */
static double stddev_stats(struct stats *stats)
{
	double variance = stats->M2 / (stats->n - 1);
	double variance_mean = variance / stats->n;
	return sqrt(variance_mean);
}