Door 1 is a lower limit and Door 4 is an upper limit.
Between these limits, a mean can be found by averaging the "time of service" of all active members.
As new members join, the mean will shift toward the upper limit.
Also, as old members leave, the mean will shift toward the upper limit.

Keep in mind, though, that both limits will constantly move up as old members leave and new members join. Eventually, assuming that all old members leave, the new members will become old members.



I believe the children are our future.