Usage data at a high-traffic web site can expose information about external events and surges in popularity that may not be accessible solely from analyses of content and link structure.

We study a simple indicator of collective user interest in an item, the batting average, defined as the fraction of visits to an item’s description that result in an acquisition of that item.

We develop a stochastic model for identifying points in time at which an item's batting average experiences significant change.

In experiments with usage data from the Internet Archive, we find that such changes often occur in an abrupt, discrete fashion, and that these changes can be closely aligned with events such as the highlighting of an item on the site or the appearance of a link from an active external referrer.