This excerpt taken from the NDAQ 10-K filed Feb 27, 2009.
Reserve for Bad Debts
The reserve for bad debts is maintained at a level that management believes to be sufficient to absorb estimated losses in the accounts receivable portfolio. The reserve is increased by the provision for bad debts which is charged against operating results and decreased by the amount of charge-offs, net of recoveries. The amount charged against operating results is based on several factors including, but not limited to, a continuous assessment of the collectibility of each account, the length of time a receivable is past due and our historical experience with the particular customer. In circumstances where a specific customers inability to meet its financial obligations is known (i.e., bankruptcy filings), we record a specific provision for bad debts against amounts due to reduce the receivable to the amount we reasonably believe will be collected. Due to changing economic, business and market conditions, we review the reserve for bad debts monthly and make changes to the reserve through the provision for bad debts as appropriate. If circumstances change (i.e., higher than expected defaults or an unexpected material adverse change in a major customers ability to pay), our estimates of recoverability could be reduced by a material amount.