December is also the least historically volatile month, with a standard deviation of returns of 3.17%.

FORBES: 5 ETFs and Stocks for a 'Santa Claus Rally'

The last column in the table is also worthy of note, as it shows the standard deviation of returns.

FORBES: Quadruple Witching Expiration Week Not So Wicked, Except For OpenTable

Sure enough, using standard deviation of the returns, the day after Election Day is significantly more volatile than a normal day by about 80% (1.99% vs. 1.14%).

FORBES: Day After Presidential Election Usually Rocky For Stocks

Proof of this can be found in the standard deviation, a measure of how volatile annual returns were.

FORBES: How To Boost Your Returns

Table 3 below provides a summary of the annualized standard deviation and returns for each asset class, the weighted average risk and return and the total portfolio risk and return from January 1979 through December 2009.

FORBES: Expert View

One common yardstick of volatility is standard deviation, which uses annualized monthly returns over time as a baseline.

FORBES: Magazine Article