Variance is a measure of how spread out a distribution is. It is computed as the average squared deviation of each number from its mean. Squaring the distance from the mean has the effect of giving greater weight to values that are further from the mean. Although the variance is intended to be an overall measure of spread, it can be greatly affected by activity at the tails of a distribution. For more information about the S&P 500 Three-Month Variance calculation, please visit the education page.