Virgil's absolutely right. Median as the "middle" value in a _sorted_ set is: - for odd number of data points, it's the middle one: set[N/2] - for even number of data points, it's the average of two in the middle: (set[N/2] + set[(N+1)/2]) / 2
Best regards, Maciej
On Tue, Aug 11, 2015 at 3:44 PM, Virgil Griffith i@virgil.gr wrote:
I mean the median.
From Wikipedia...
For example, if *a* < *b* < *c*, then the median of the list {*a*, *b*, *c*} is *b*, and, if *a* < *b* < *c* < *d*, then the median of the list { *a*, *b*, *c*, *d*} is the mean of *b* and *c*; i.e., it is (*b* + *c*) / 2.
-V
On Tue, Aug 11, 2015 at 9:29 PM John oneofthem@riseup.net wrote:
I think you are confusing the median with the mean:
https://en.wikipedia.org/wiki/Median https://en.wikipedia.org/wiki/Mean
Taking the median instead of the mean can be beneficial in situations where you have larger outliers in your data, which typically affect the mean very much.
-j
Virgil Griffith:
Is there some implementation-specific reason not to use the standard mathematical definition of "median"? If not, I propose changing the implementation to become it.
-V
On Tue, Aug 11, 2015 at 2:44 AM Nick Mathewson nickm@alum.mit.edu
wrote:
On Mon, Aug 10, 2015 at 1:11 PM, nusenu nusenu@openmailbox.org
wrote:
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Hi,
https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2028
If 3 or more authorities provide a Measured= keyword for a router, the authorities produce a consensus containing a "w" Bandwidth= keyword equal to the median of the Measured= votes.
a random sample from recent votes:
grep 37.59.38.117 -A 3 *|grep Measured w Bandwidth=6869 Measured=7570 w Bandwidth=6869 Measured=15500 w Bandwidth=6869 Measured=18100 w Bandwidth=6869 Measured=30500
Tor says the median value is 15500
2015-08-10-16-00-00-consensus: w Bandwidth=15500
but the median of these 4 values is actually: (18100+15500)/2 = 16800 no?
Has tor a different definition of 'median' and simply takes always the second ordered measurement vote out of 4 votes or is there a bug in the spec or implementation?
There's one misplaced throwaway sentence in dir-spec.txt:
" All ties in computing medians are broken in favor of the smaller or earlier item. "
We should bring this, and probably other things, into a "definitions" section earlier in dir-spec.txt. Patches welcome. ;)
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