Questions and answers about user statistics
===========================================

Q: How is it even possible to count users in an anonymity network?
A: We actually don't count users, but we count requests to the directories
that clients make periodically to update their list of relays and estimate
user numbers indirectly from there.

Q: Do all directories report these directory request numbers?
A: No, but we can see what fraction of directories reported them, and then
we can extrapolate the total number in the network.

Q: How do you get from these directory requests to user numbers?
A: We put in the assumption that the average client makes 10 such requests
per day.  A tor client that is connected 24/7 makes about 15 requests per
day, but not all clients are connected 24/7, so we picked the number 10
for the average client.  We simply divide directory requests by 10 and
consider the result as the number of users.  Another way of looking at it,
is that we assume that each request represents a client that stays online
for one tenth of a day, so 2 hours and 24 minutes.

Q: So, are these distinct users per day, average number of users connected
over the day, or what?
A: Average number of concurrent users, estimated from data collected over
a day.  We can't say how many distinct users there are.

Q: Are there more fine-grained numbers available, for example, on the
number of users per hour?
A: No, the relays that report these statistics aggregate requests by
country of origin and over a period of 24 hours.  The statistics we would
need to gather for the number of users per hour would be too detailed and
might put users at risk.

Q: Are these tor clients or users?  What if there's more than one user
behind a tor client?
A: Then we count those users as one.  We really count clients, but it's
more intuitive for most people to think of users, that's why we say users
and not clients.

Q: What if a user runs tor on a laptop and changes their IP address a few
times per day?  Don't you overcount that user?
A: No, because that user updates their list of relays as often as a user
that doesn't change IP address over the day.

Q: How do you know which countries users come from?
A: The directories resolve IP addresses to country codes and report these
numbers in aggregate form.  This is one of the reasons why tor ships with
a GeoIP database.

Q: Why are there so few bridge users that are not using the default OR
protocol or that are using IPv6?
A: Very few bridges report data on transports or IP versions yet, and by
default we consider requests to use the default OR protocol and IPv4.
Once more bridges report these data, the numbers will become more
accurate.

Q: Why do the graphs end 2 days in the past and not today?
A: Relays and bridges report some of the data in 24-hour intervals which
may end at any time of the day.  And after such an interval is over relays
and bridges might take another 18 hours to report the data.  We cut off
the last two days from the graphs, because we want to avoid that the last
data point in a graph indicates a recent trend change which is in fact
just an artifact of the algorithm.

Q: But I noticed that the last data point went up/down a bit since I last
looked a few hours ago.  Why is that?
A: The reason is that we publish user numbers once we're confident enough
that they won't change significantly anymore.  But it's always possible
that a directory reports data a few hours after we were confident enough,
but which then slightly changed the graph.

Q: Why are no numbers available before September 2011?
A: We do have descriptor archives from before that time, but those
descriptors didn't contain all the data we use to estimate user numbers.
Please find the following tarball for more details:

  https://metrics.torproject.org/data/old-user-number-estimates.tar.gz

Q: Why do you believe the current approach to estimate user numbers is
more accurate?
A: For direct users, we include all directories which we didn't do in the
old approach.  We also use histories that only contain bytes written to
answer directory requests, which is more precise than using general byte
histories.

Q: And what about the advantage of the current approach over the old one
when it comes to bridge users?
A: Oh, that's a whole different story.  We wrote a 13 page long technical
report explaining the reasons for retiring the old approach.  tl;dr: in
the old approach we measured the wrong thing, and now we measure the right
thing.

  https://research.torproject.org/techreports/counting-daily-bridge-users-2012-10-24.pdf

Q: What are these red and blue dots indicating possible censorship
events?
A: We run an anomaly-based censorship-detection system that looks at
estimated user numbers over a series of days and predicts the user number
in the next days.  If the actual number is higher or lower, this might
indicate a possible censorship event or release of censorship.  For more
details, see our technical report.

  https://research.torproject.org/techreports/detector-2011-09-09.pdf

