Given the large number of articles transferred between servers and the large size of individual articles, their complete propagation to any one server farm is not guaranteed. The term "completion" is used to describe how well a service is keeping up with the traffic.
The primary obstacle to calculating the completion percentage is how many articles were posted. Looking at only one server, one cannot know how many articles were actually inserted throughout the network. Articles may never make their way outside the originating server, or may fail to find their way out to the transit cloud. Very large articles are frequently dropped, and tend to propagate less well than smaller ones.
One way to measure completion is to access multiple servers and retrieve lists of articles. Because Message-ID: headers are nominally unique throughout the network, comparison of the lists is mostly a straightforward task. Practical limitations to this type of measurement include the impossibility of obtaining lists from all servers worldwide, the fact that many servers filter out spam or employ Usenet Death Penalties, and that some servers mask incompletion by hiding multipart binary sets with missing articles. It is also necessary to take into account propagation times and retention; an article may simply have not yet arrived at a given server, or it may have been present but already expired.
Read more about this topic: News Server Operation