Intrusion Prevention Systems (IPS testing) require constant testing in order to verify the device is working properly. Every new software upgrade and/or signature set completely changes the way an IPS will perform in your network. In all cases, the data derived from any test must be objective, qualitative, and consistent.
A new generation of content-aware network equipment has forced testing to move up the stack to more realistic Layer 4-7 IPS testing. You simply cannot rely on the same tools and methodologies once used to test routers and switches for IPS testing.