"David, I have a question regarding fast computerized trading of stock markets. I recently heard estimates that the "algos" account for more than 50% of NYSE volume and that most of this increase has occurred since the 2008 crash. Now a glance at Bloomberg weekly charts shows about 6 billion shares trading per week vs about 4 billion more recently (what one would expect in a bear market rally if such is the case). My question though is how the algos can account for >50% of all trading given the volume numbers cited above. Unless the algos have been active for a much longer time, "normal" investors and traders would have largely departed the market. Perhaps ultra short term trades, most of which never show up at the end of the day, are not counted in the volume totals?

"Perhaps someone in the collective has an idea about this."

David Fuller's view Thank you for a very interesting question. However, it is not one that I am qualified to answer although someone within the Collective - particularly those working within the bigger institutions - will have insights on this subject.

Personally, volume is not an indicator that I use very often, particularly regarding indices. However, you can add it in the 'Charting' link above each graph and save a volume template if you wish. For the purposes of discussion I have included volume on weekly and month charts of the S&P 500 Index, although our volume data for this only goes back to 1993.

The volume trend has certainly been lower since the climactic 2002 lows. The figures I hear for HFT are up to 80% of daily trading but this is not something that I can calculate. I would not be surprised if a proportion of HFT volume was not reported. This is an additional concern with volume as an indicator because we sometimes hear that 'dark pools' now account for more turnover than official exchanges. If so, while volume has been going down on the S&P as officially calculated, it might have actually risen in aggregate.