The Monthly Volatility number is a measure of how much the value of a portfolio moves up and down from month to month. Technically, the monthly volatility numbers listed in the table are 1 standard deviation of the percentage changes that have occurred in each portfolio since 1992.

Essentially, what this means is that 2/3 of the time, you can expect the monthly percent change in a portfolio to be within plus or minus the value listed. Many financial engineers equate the monthly volatility to risk - the higher the volatility, the higher the risk. However, because some volatility is good (every investor likes their portfolio to go up fast), financial engineers use the Sharpe ratio as a better overall gauge of risk versus reward. The Sharpe Ratio for each portfolio is also listed Performance Statistics table.