Spam Filter Batting Average

[http://jgc.org/ John Graham-Cumming] proposed this uniform measure of spam-filter effectiveness in [http://www.jgc.org/antispam/11162004-baafcd719ec31936296c1fb3d74d2cbd.pdf his November 16, 2004 article entitled 'Understanding Spam Filter Accuracy'].

Essentially, it's a reformatting of the FalsePositive percentage and FalseNegative percentage, as 'spam hit rate / ham strike rate'. This can be computed from FP%/FN% as follows:

  let fp = false positive percentage
  let fn = false negative percentage
  batting average hitrate = (1 - (fn / 100))
  batting average strikerate = (fp / 100)
  batting average = "hitrate/strikerate"

so if you have an FP% of 0.03%, and an FN% of 2.47%, the batting average is

  (1 - (2.47 / 100)) "/" (0.03 / 100) =
  .9753/.0003

That's actually the correct batting average for SpamAssassin 3.0.0's scoreset 3, measured against the validation corpus when we released it.

See also MeasuringAccuracy for other schemes used, or FpFnPercentages for the main one we use in SpamAssassin.