Here are @Quiddity's comments from the team discusion on this:

We should specify which definitions those are, and why they aren't appropriate, in citations/footnotes.
Links I know of that might be relevant include:

• https://meta.wikimedia.org/wiki/Research:Metrics_standardization#New_editor

(Question1: are these actually fully standardized across the Org yet?)
(Question2: are the other metrics listed there, also unsuitable for our purposes?)

and the 2 different RC options

• https://en.wikipedia.org/wiki/Special:RecentChanges/Newbies

and

• https://en.wikipedia.org/w/index.php?title=Special:Contributions&contribs=newbie

(1 of these is from ~2006, and we were told it is badly calibrated. Can we fix that whilst we're poking at these things?
Where does the other one come from? - please document in https://www.mediawiki.org/wiki/Help:Recent_changes )

Existing definitions

• For research purposes, @Halfak and @DarTar define a "New Editor" as a "Newly Registered" user who has completed 1 edit within 1 day.
• On the Newbies page, "Newbies" are the most recent 10 % of any wiki (though I'm not sure this is actually documented anywhere; I got this info from @Catrope, who looked at the code).

Discussion
Neither of these definitions seem relevant to our purpose, and I think we can proceed. If anyone knows about any other definitions we need to be aware of please speak up.

Unregistered users

For our purposes, which have to do with identifying and supporting those who might need help, I am assuming that we are equally interested in Registered and Unregistered users. (If reviewers want to avoid unregistered users, that can be achieved via filters.) Two questions:

• Though IP address identification is imperfect, is there any technical reason why we wouldn't attempt to identify and help IP users through this means?
• Is there any policy or practical reason why we wouldn't want to include IP users among our Newcomers?

I asked:

For our purposes, which have to do with identifying and supporting those who might need help, I am assuming that we are equally interested in Registered and Unregistered users.

@Halfak says that for a variety of reasons, it's not helpful to follow unregistered users (one issue has to do with cellphone IP addresses, which are apparently not stable).

With that, we have approved definitions and wording for three levels. I have one last question though: These are all Registered users. It seems like we should signal that somehow, maybe in the subheading (so it doesn't have to be repeated 3 times). E.g., see below:

User experience level (registered users)

Newcomer
Very new editors: fewer than 10 edits and 4 days of activity.

Experienced
More days of activity and edits than “Newcomers” but fewer than “More experienced.”

More experienced
More than 30 days of activity and 500 edits.

@Pginer-WMF, wanted to make sure you saw my suggestion above, to signal that all the "User experience level" filters will apply only to registered users. I was thinking to add this in the section title:

User experience level (registered users)

or

User experience level (registered users only)

In T145157#2657649, @jmatazzoni wrote:

@Pginer-WMF, wanted to make sure you saw my suggestion above, to signal that all the "User experience level" filters will apply only to registered users. I was thinking to add this in the section title:

User experience level (registered users)

or

User experience level (registered users only)

I updated the prototype to reflect this. I used "User experience level (for registered users)" to keep the clarification connected with the rest of the title. This group is also placed right below the "User registration" which may also help to clarify the connection.

I recorded our decisions in the task description and am now closing this task as complete.

In T145157#2621668, @jmatazzoni wrote:

and the 2 different RC options

• https://en.wikipedia.org/wiki/Special:RecentChanges/Newbies

I don't think /Newbies has an effect. Compare https://www.mediawiki.org/wiki/Special:RecentChanges/Newbies to https://www.mediawiki.org/wiki/Special:RecentChanges .

• https://en.wikipedia.org/w/index.php?title=Special:Contributions&contribs=newbie

This one works, though.