This paper compares several statistical models for monthly stock return volatility. The focus is on U.S. data from 1834-19:5 because the post-1926 data have been analyzed in more detail by others. Also, the Great Depression had levels of stock volatility that are inconsistent with stationary...

"This paper uses monthly returns from 1802-2010, daily returns from 1885-2010, and intraday returns from 1982-2010 in the United States to show how stock volatility has changed over time. It also uses various measures of volatility implied by option prices to infer what the market was expecting...