1. A computer-implemented method of reporting on a measured change in a consumption rate at a reporting system, the computer-implemented method comprising: receiving a respective media consumption history of each of a plurality of entities; selecting a monitored audience from the plurality of entities by selecting entities which have completed an online purchase; determining a category value for each of two or more categories of media content wherein: one category of media content comprises one or more items of media content; one category value represents the measured change in consumption rate for one category of media content for the monitored audience between at least two time intervals; and one consumption rate is determined based on the number of times each item of media content in the one category has been delivered over a network during one time interval, at the reporting system; and providing a report comprising a reference to two or more categories and their associated category values.