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 0.1%, and an FN% of 2.5%, the batting average is

  (1 - (2.5 / 100)) "/" (0.1 / 100) =
  .975/.001

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