[metrics-bugs] #26022 [Metrics/Statistics]: Fix a flaw in the noise-removing code in our onion service statistics

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#26022: Fix a flaw in the noise-removing code in our onion service statistics
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 Reporter:  karsten             |          Owner:  metrics-team
     Type:  defect              |         Status:  needs_review
 Priority:  Medium              |      Milestone:
Component:  Metrics/Statistics  |        Version:
 Severity:  Normal              |     Resolution:
 Keywords:                      |  Actual Points:
Parent ID:                      |         Points:
 Reviewer:                      |        Sponsor:
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Comment (by amj703):

 Hey Karsten,

 (My trac pseudonym is amj703 instead of ohmygodel. I've changed the cc).

 I agree you did find a bug in how the noisy numbers are adjusted. The
 change from integer division to floorDiv seems right to me. One thing you
 might also consider doing to improve handling negative values is to
 disallow them (i.e. round up to zero). This could be done by the relay
 reporting its number. We probably discussed this option, and maybe the
 reason it wasn't chosen is that it slightly biases counts so their
 expected value isn't the true value (or at least the true rounded value).
 If that's the case, and negative values are allowed to prevent biasing, we
 should recognize that (1) values are already being biased because of the
 rounding (for which we minimize the worst-case bin by adding (-binSize/2)
 at the end), and (2) adding (-binSize/2) actually makes the bias worse for
 negative bin values.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26022#comment:7>
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