no of good emails classified as spam / total number of good emails.
E.g. if Your mail filter classified 100 emails as 90 spam and 10 good and of the 90 classified as spam, 2 are good, your false positive rate = 2/10 *100% = 2%
This is an important measure because it measures are likely you are to wrongly junk a good email. If you have a false positive rate of 2% it means if you trust the filter fully, and don't trust for mistakes, you will lose 2 good emails wrongly classified as junk spam out of every 100 good emails.
Edited by Paranoid, 06 July 2004 - 10:50 AM.