1. A computer implemented method of identifying user propensity to access content via a communication network, comprising: receiving, at a data processing system, from a client device via the computer network, a request for content to provide for display with a destination web page via a Hypertext Transfer Protocol (HTTP); identifying, by the data processing system, from a HTTP header field of the request, a referring URL that identifies a source web page, the source web page different from the destination web page and requested by the client device prior to requesting the destination web page; determining, by the data processing system, an amount of time passed since the source web page was accessed by the client device based on browsing history of the client device; determining, by the data processing system, a web site category of the source web page based on the referring URL identified from the HTTP header field of the request; estimating, by the data processing system, a probability that the client device accesses content on one or more destination web pages after requesting the source web page based on the amount of time passed since the source web page was accessed by the client device and the web site category of the source web page; assigning, by the data processing system, a score to the request based on the probability that the client device accesses the content on the one or more destination web pages after requesting the source web page; adjusting, by the data processing system, a first quality metric for the client device based on the score, the first quality metric indicating a decreased likelihood, relative to a prior quality metric for the client device, of the client device to access a first content item provided for display with the destination web page; and transmitting, by the data processing system, responsive to adjusting the first quality metric, a second content item to the client device for display with the destination web page, the second content item different from the first content item and corresponding to a second quality metric greater than the first quality metric.