For a pretty good analysis of problems with this, see Brad DeLong's take: <a href="http://delong.typepad.com/sdj/2012/07/equity-returns-and-the-size-of-the-economy-bill-gross-makes-a-distressingly-common-error.html" rel="nofollow">http://delong.typepad.com/sdj/2012/07/equity-returns-and-the...</a>