When looking around for examples of how five levels of defect severity are named it turned out that almost everybody seem to use four levels of severety (they name them differently though).
Is there a rational reason for this? Any fact that says that this is actually the best thing to do?
Or is it just that everybody does this because everybody else does it? Which means that it's a reasonable idea to (go against the flow and) use five levels instead.
As I mentioned I've got no great idea what to call these five levels (perhaps critical, major, medium, minor, trivial or something). The idea is that the highest of course is catastrophic/critical/blocker (ie something that we simply can't stand or nearly so) then the lower would be geometrically smaller in expected impact. Where it ends is when the expected impact is too low to bother about.