Hi
Fragment of a discussion from Talk:Editor Trends Study
Maybe bots that are linking articles between different project have an implication on this study. You could detect them via Unified Login, because there you can see which is the home wiki. Or you are able to see if a user has a bot flag, but I don´t know if every bot has a bot flag.
By the way, in your definition of New Wikipedians and Active Editors you don´t say anything about bots and editors with a different home wiki.