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Where can I find information on the percentage of volume is contributed by shorts?
I believe that it's not possible for the public to know what shares are being exchanged as shorts because broker-dealers (not the exchanges) handle the shorting arrangements.  I don't think exchanges can even tell the difference between a person selling a share that belongs to her vs. a share that she's just borrowing.  (There are SEC regulations requiring some traders to declare that trades are shorts, but (a) I don't think this applies to all traders, (b) it only applies to the sells, and (c) this information isn't public.) That being said, you can view the short interest in a symbol using any of a number of tools, such as Nasdaq's here.  This is often cited as an indicator similar to what you proposed, though I don't know how helpful it would be from an intra-day perspective.