This excerpt taken from the NDAQ 8-K filed Feb 20, 2008.
Highly recurring revenue base
The majority of The NASDAQ OMX Groups net exchange revenues will be generated by activities that are not driven by trading volume. On a pro forma basis for the 12 months ended September 30, 2007, 67.6% of our combined net exchange revenues were fee-based, including revenues from annual listing fees, data and financial products, shareholder, director and newswire services, as well as OMXs technology solutions business. On a pro forma basis for the 12 months ended September 30, 2007, OMXs technology solutions business represented 16.0% of the net exchange revenues of The NASDAQ OMX Group. This revenue stream is highly recurring since the contracts are for multiple years and the annual revenues are typically fixed upfront. On a pro forma basis for the 12 months ended September 30, 2007, 32.4% of our combined net exchange revenues were transaction-based. However, we believe that a significant portion of our net exchange revenues related to trading is also recurring since we believe that a core level of trading activity always exists, irrespective of market conditions. For example, in the last 15 years, Nasdaq-listed average daily volume has never dropped by more than 10% from one year to the next.