ROC Graphs

Wiki Markup\[http://en.wikipedia.org/wiki/Receiver-Operator_Characteristic Wikipedia says\]:

In signal detection theory, a receiver operating characteristic (ROC) is a graphical plot of the sensitivity vs. 1-specificity for a binary classifier system as its discrimination threshold is varied. The ROC can also be represented equivalently by plotting the fraction of true positives (TP) vs. the fraction of false positives (FP). The usage receiver operator characteristic is also common.

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In SpamAssassin terms, a ROC graph is the FpFnPercentages values, graphed over multiple thresholds. There's a script in SVN used to measure this at masses/mk-roc-graphs; here's sample output:

http://taint.org/xfer/2005/roc_curves_with_3045.png

See also MeasuringAccuracy for other methods used to measure spamfilter accuracy.