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S/O stands for "spam / overall", in other words, the proportion of the total hits that were spam messages. As such, it is equivalent to Bayesian probability, or 'positive predictive value' in pattern discovery Positive Predictive Value in bioinformatics or medicine.

Measuring Rule Overlap

There's one more tool to determine how much 2 rules overlap with each other – "overlap". This is occasionally useful if you suspect that two rules are redundant, checking the same data or hitting exactly the same messages as each other. Take a look at the comments at the top of the "masses/overlap" script for details on how to run this against one or more "mass-check" output log files.

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