Here is a daily chart of the S&P 500 showing a good part of the last bull market, the 2007-09 bear market, and the rally (which I think is the start of a new bull market) from the March '09 lows. I am posting this chart because of a comment just made by John M:
It is my view that most people do not extract useful information from volume statistics because they don't make the correct comparisons. They usually look at the trend of volume within a rally or decline. But I think this is the wrong thing to look at. Instead the volume on rallies must be compared with volume on previous rallies and the volume on declines must be compared with volume on previous declines.
The relevant comparison for the current rally's volume is to the volume during rallies on the way down and to the volume during the 2002-2007 bull market. On both counts the volume on the rally from the March 6 low has been very high. Therefore I think that the volume has been telling us that a lot of aggressive, long term buying has been propelling the rally from the March lows. This is one thing that is telling me that a new bull market has started.