Google's move to keep logged in user traffic information private at the end of 2011 can induce heart attacks when comparing year over year web stats.
Google has made year over year comparisons in Google Analytics a true pain with their SSL (Secure Socket Layer) "protection" of data from users logged into their Google accounts when they visit your websites. When you see "Not Listed" in your GA it is really "hidden" data from those logged into their acounts. We've seen "Not Listed" reach 40% and it has been gaining monthly.
This quick article will take you through a down and dirty way to model any data you need to have a year over year comparison. There are 3 easy steps and this quick back of the napkin process may save you pain and a heart attack since 2012 data looks REALLY BAD until you adjust the numbers by adding the SSL data BACK in to make sure you are comparing apples to apples instead of apples to zebras.