Find a good metric for bridge churn
We are still lacking a good metric for bridge churn, that is, the rate of joining and leaving bridges over time. If this rate is too high, we should give out more bridges to bridge users, because otherwise they'll soon end up with bridges that left the network. If the rate is lower than expected, we might even give out fewer bridges, and bridge users will still be able to use Tor.
Here's a possible metric for bridge churn: Of the bridges that are running at the beginning of a month, what fraction keeps running throughout the month? And what fraction of bridges keeps running on the same IP address that they had at the beginning of the month?
In the attachment you'll find a graph for January 2011. The graph shows when bridges that were running on January 2 are listed in the bridge network status throughout the month with a) any IP address or b) the same IP address. I'm going to commit the code to metrics-tasks and add a link here later today.