The financial analogy is to think of failure rates in drives as equivalent to defaults on a pool of securities (say mortgages). obviously there are a number of ways to do this but the most simple is that since origination standards for loans vary through time (think build quality and design changes in the drive context) it seems to make sense to think about this from a vintage standpoint. i.e. are seagate drives installed in Dec'12 failing at the same rate as ones installed in March '13 after six months...
