This excerpt taken from the WBMD 10-K filed Mar 16, 2006.
DEFINITIONS OF CERTAIN MEASURES OF USAGE OF THE WEBMD HEALTH NETWORK
In this Annual Report, we provide information regarding usage of The WebMD Health Network that we have determined using internal technology that identifies and monitors usage by individual computers. As used in this Annual Report:
Third party services that measure usage of Internet sites may provide different usage statistics than those reported by our internal tracking technology. These discrepancies may occur as a result of differences in methodologies applied and differences in measurement periods. For example, third party services typically apply their own proprietary methods of calculating usage, which may include surveying users and estimating site usage based on surveys, rather than based upon our internal tracking technology. Regardless of the measure of usage, if usage data is prepared using a consistent methodology between measurement periods, we believe that it provides useful information over time.