1. A method for determining global impression volumes, the method comprising performing at a processor coupled to a memory and in communication with a communication interface, the steps of: receiving in the memory, via the communication interface, from a selected panel of computers, panel impression data associated with a plurality of tags, at least one tag being a reference tag; preprocessing the received panel impression data to obtain normalized impression data for each tag in the plurality of tags, the preprocessing comprising reconstruction to balance an inaccuracy associated with a first privacy setting, in the panel impression data; receiving in the memory, via the communication interface, global impression data from at least one computer distinct from any computer in the selected panel, the global impression data comprising global reference tag data for the reference tag, and impression data associated with each tag comprising impression volume of that tag, representing a number of instances of that tag that were loaded; computing an upscaling factor based on, at least in part, a ratio of the global reference tag data to the normalized impression data associated with the reference tag; and for a specified tag within the plurality of tags, computing global impression volume based on, at least in part, the upscaling factor and the normalized impression data associated with the specified tag.