Excellent question. It is simply the sum of all the times
a user has posted on Metafilter (comments or posts in any section of MeFi)
divided by the number of days they have been a member.
Tamim's user
profile discusses the idea and while reading
this discussion I saw
moz's reference
to tamim's user page. After reading that and seeing
tamim's suggestion,
I wrote a quick script to do it.
If you see any problems please let me know.