[Wikimedia-l] User retention statistics?

[Yaroslav M. Blanter]

My message is inspired by discussion in this thread 
(http://en.wikipedia.org/wiki/Wikipedia:Administrators%27_noticeboard#Loss_of_more_and_more_and_more_established_editors_and_administrators) 
on Englush Wikipedia. Whereas the thread itself is not relevant to this 
list, and the points get re-iterated on a regular basis, there were 
statements made there which contain quantitative estimates (for instance 
that 90% established users who leave do it because they get a new job or 
have their external life changed in some other way, and not because of 
harassment etc). Most probably these numbers are not really justified, 
but then I wanted to know what real numbers are. I am an Rcom member, 
but I can not recollect such research being accomplished (I might be 
wrong of course). I could not find data easily either (I spent half an 
hour because I remembered we had a Community Health initiative group 
which somehow evolved into the Movement Roles, but the Movement Roles 
pages on Meta do not talk about community health at all, and I could not 
even find an appropriate page to ask the question).

After this long introduction, does somebody know / can point out the 
answers to the questions:

1. What is the average lifetime of a Wikipedia editor (for instance the 
one with at leat 1000 contributions)? I recollect smth about two years, 
but I am pretty sure I have never seen any research on this. How does it 
depend on the number of contributions?

2. What are the main reasons why these editors stop editing? Is this 
correct, for instance, that external reasons are much more important 
than internal (on-wiki troubles and wiki-related harassment) reasons? 
The same for say those above 10000 edits?

Thanks in advance
Cheers
Yaroslav