Wikipedia, being a non-sentient encyclopedia, has done none. The Wikimedia Foundation’s Research Team may have done (You can ask on the discussion page[2].), or may have facilitated such research by third parties[3]. [1] https://meta.wikimedia.org/wiki/Research:Index [2] https://meta.wikimedia.org/wiki/Research_talk:Index [3] For example: https://meta.wikimedia.org/wiki/Research:Characterizing_Wikipedia_Citation_Usage