  Wikipedia, as a social phenomenon of collaborative knowledge creating, has
been studied extensively from various points of views. The category system of
Wikipedia, introduced in 2004, has attracted relatively little attention. In
this study, we focus on the documentation of knowledge, and the transformation
of this documentation with time. We take Wikipedia as a proxy for knowledge in
general and its category system as an aspect of the structure of this
knowledge. We investigate the evolution of the category structure of the
English Wikipedia from its birth in 2004 to 2008. We treat the category system
as if it is a hierarchical Knowledge Organization System, capturing the changes
in the distributions of the top categories. We investigate how the clustering
of articles, defined by the category system, matches the direct link network
between the articles and show how it changes over time. We find the Wikipedia
category network mostly stable, but with occasional reorganization. We show
that the clustering matches the link structure quite well, except short periods
preceding the reorganizations.
