1. A computer-implemented method of measuring and analysing multiple data sources over a communications network in order to ascertain information about use of one or more resources linked to said communications network, said method comprising: obtaining, by a server, site-centric data, wherein the site-centric data corresponds to a first group of one or more monitored resources; obtaining, by the server, user-centric data, wherein the user-centric data corresponds to a second group of one or more monitored resources; combining, by the server, the site-centric data and the user-centric data to form a single data source corresponding to usage information of one or more resources; processing, by the server, the site-centric data and the user-centric data to form a calibration value; and calibrating, by the server, an unmonitored resource value corresponding to one or more unmonitored resources based on the calibration value by multiplying the unmonitored resource value by the calibration value; wherein the processing to form a calibration value comprises: calculating a weighting factor based on a number of users that have interactions recorded in relation to the second group of one or more monitored resources and a total number of users expected to have access to the one or more resources available through the communications network; and multiplying the weighting factor with a number of users in the sample group that have interactions recorded in relation to the first group of one or more monitored resources to obtain a first figure for an expected number of all users to have interactions with the first group.