4. Combining FPs and FNs Suppose we were evaluating two filters, Filter A and Filter B. Filter A has a catch rate of 91% but an FP rate of 5%. Filter B has a catch rate of 75% but an FP rate of 2%. Which is better? How can we combine the two metrics? One way to do it is to create a relative performance index. Simply divide the catch rate by the FP rate. As the FP rate increases, the RPI gets smaller (meaning the filter is performing worse). As the Catch rate increases, the RPI gets larger (meaning the filter is performing better).
Relative Performance Index= Catch rate / FP rate
SITI = = Spam messages in user′s inbox / Total messages in user′s inbox = FN / (TN+FN)
Apparently it's also like an episode of The Sopranos around there.