1. In a computer network that includes (i) a plurality of servers for accessing a plurality of network sites containing various types of content, as for example text, graphics and audio files, which can be viewed and listened to as appropriate, and downloaded when desired, and that includes (ii) a plurality of client systems each including a processor and input device for permitting connection to said network for purposes of browsing the network sites, a method of deriving user information such as age, identity or credit worthiness, about the users from one or more of the client systems as the network sites are browsed, and of associating the derived user information with a level of certainty regarding the accuracy of the derived information, the method comprising: a step for monitoring on-line activity of a user as a client system is connected to the network as the user is browsing one or more sites on the network, said monitoring function comprising: an act of parsing information received at a client system for a particular site and monitoring the parsed information the parsed information input to the client system; and an act of monitoring information output from the client system to one or more of the network servers; a step for storing the monitored on-line activity about the user in a set of information fields, said step for storing the monitored on line activity comprising: an act of associating each information field in the set of information fields with a status field, the status field indicating a level of certainty regarding the accuracy of information contained in the corresponding information field; an act of, for a first information field in the set of information fields, determining whether a source of data exists that is more reliable than information currently contained in the first information field based upon a first status field corresponding to the first information field; an act of retrieving information corresponding to the first information field from the source of data if it is determined that a more reliable source of data exists; and an act of replacing the information currently contained in the first information field with the retrieved information.