You get what you measure - so how to create key values on bugs

Bugs are elusive. Not only that they occur, but that it seems that most key values which measures bugs is contraproductive but reading about Yahoo and their transition to agile, I've found a new interesting metric: number of critical bugs found in X within Y days after placed in Test/Production/etc. For example if you have the metric "The number of critical bugs found in production within 5 days of production".

Why is this interesting? Well, if it took the users less than five days to find the problem, the software development team should have found it as well.

The same type of metrics should be applicable in the different stages of a sprint, for example number of critical bugs found the first day of acceptance testing.

There is probably a downside to this metric but so far I like it