What I thought was most insightful was the chart with all the recessions dating back to 1899-1900. That chart showed the average of the 21 recessions excluding the Great Depression to be -2.7% of GDP. Including the Great Depression you get
-3.7% of GDP for the average economic correction.
I've thought for some time we could see a 10% decline in a double dip recession in 2008-2009 and 2010-2011. If you look at the first half of the century, there were many double dip recessions. The most recent one was 1980 and 1981-1982. We're due.
Shilling suggests one recession with a 5% pullback. However, with stimulus running out in 2009 and mortgage foreclosures ratcheting up in 2010 and 2011, I don't see how we get only one recession.
Of note, the 2nd recession of the double dip is typically much worse than the first.