A key feature of any anti-spam solution is how effective it is at removing spam. A perfect anti-spam system would have a zero false positive rate and a zero false negative rate. In practice, this is not usually achieved, and systems will invariably trade off the two measurements.
A new whitepaper on the Isode website describes how false negatives can be measured and looks at false negative rates from the beginning of this year for Isode's M-Switch Anti-Spam.
The graph below shows the false negative rate from January 2008.