Below is a response i got to my question about estimating average life
of a player (replicated below -
http://answers.google.com/answers/threadview?id=568774)  - See my new
quetsion below:
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From: hfshaw-ga on 16 Sep 2005 16:46 PDT     
For simplicity, let's assume that people only join or drop out on the
first day of each month, and calculate the average "age" of the
players present on the last day of each month.

Then, at the end of month 1, there were 100 players, all of whom had
been playing for 1 month, so the average age of the player would have
been 1 month.

At the end of month 2, there were 75 players who had been playing for
1 month, and 90 who had been playing for 2 months.  The average "age"
would then be (75*1 + 90*2)/(70+90) = 1.55 months. At the end of month
3, there were 120 players who had been playing for
1 month, 60 who had been playing for 2 months, and 50 who had been
playing for 3 months.  The average "age" would then be (120*1 + 60*2 +
50*3)/(120+60+50) = 1.70 months.

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NEW QUESTION/ISSUE:
By this methodology, is it only going to increase every month?
In what cases can it actually decrease?
I have a set of 50 months that i want to analyze. They show average
life increase every month by the above methodology.
From people who started in early months, there is a leveling off to
about 30% from originals (i.e. 100 in month 1 and 30 stayed in month
50). There is a noticeable increase in new people in recent months.
In simple words - what is the logic for the average life of a player to go up?
Should it not be declining? Under what circumstances will it go down?
Thanks.