At the extreme, we've seen in excess of 25,000 quote changes in a single stock in one second of time or less (this page has a chart that documents every extreme example). Often there are no trades during these events. Sometimes a simple pattern evolves from the quote price changes, such as in the case of a certain High Frequency Trading (HFT) algorithm that we've recently seen run every day in Google stock.

This particular algorithm starts with a bid (or offer) several dollars away from the bids (offers) from one of the other 10 exchanges trading Google Class A stock (symbol GOOGL). We've also seen this algo running in other higher priced stocks. The algo in this example only appears to run from the Nasdaq-Boston (BOST) exchange. In the chart below, we show bids and offers color coded by reporting exchange (there are 10 exchanges in GOOGL). Note that these are "top of book" quotes - that is, they are the highest bid price and lowest ask price from that exchange.