S&P 500 Trading Volume Recent History

Even as the Dow looks like it could post back to back 200 point increases, there are signs that equity markets are still structurally quite ill.  Aggregate volume in S&P 500 component stocks continues to decline.  While summer is always a light volume period seasonally, each of the last three summers have seen lower lows.  Price doesn't necessarily need volume for confirmation, but it is worrisome that there are fewer and fewer participants in the equity markets.