1. A computer-implemented method for analyzing responses to challenge graphics, comprising: monitoring, by a third party computer system, responses to a plurality of challenge graphics, the plurality of challenge graphics comprising a first challenge graphic requested by a first publisher computer system to determine whether to grant access to a first resource in connection with a first transaction on a first web site hosted by the first publisher computer system, the plurality of challenge graphics further comprising a second challenge graphic requested by a second publisher computer system to determine whether to grant access to a second resource in connection with a second transaction on a second web site hosted by the second publisher computer system, the second web site being different from the first web site; associating each response with a same network address identifying at least one user computer system to which the plurality of challenge graphics are served; measuring, by the third party computer system, at least one characteristic of the responses to identify a pattern, wherein: the at least one characteristic relates to a duration of an event; and the pattern relates to a number of responses for which the duration of the event is below a selected threshold; and providing, by the third party computer system, based at least in part on the identified pattern, score information in association with the network address, the score information indicative of a level of trustworthiness of the at least one user computer system to which the plurality of challenge graphics are served.