As there's no central point on Usenet, it is impossible to provide canonical statistics. However, having numbers such as the activity of authors, number of messages, amount of data posted for a given hierarchy or even Usenet as a whole from various points and comparing them usually gives a good idea on what's happening across Usenet. The fact that several well-known news server applications already generate such data for maintenance purposes and their ability to display them graphically in HMTL files makes the task even more easier.
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