I'm trying to plot the 30 day historical volatility of the S&P 500,
using the following AFL on a daily chart with around 2 years of end of day
prices for the S&P 500:
Plot(StDev(Close, 30),"Historical
Volatility", colorOrange,styleLine);
However when I look at the values,
they appear to be a bit off to what I see when I look online
here:
http://www.ivolatility.com/options.j?ticker=SPX:CBOE&R=1&period=12&chart=2&vct=For
today they have 18.99, where as my chart shows 19.348. All my other values
going back further don't match their chart either. Any ideas, are they
calculating it differently than just a simple standard deviation?