The Tier Rating System, developed and implemented by The Uptime Institute (which itself is now part of The 451 Group) has been the standard measure bantered about by clients seeking data center space.
Over time it has been supplemented by broad interpretations, no doubt to the at least ocassional dismay of its developers. These interpretations have been somewhat of a necessity given the lack of flexibility written into the standard.
For example, each tier specifies a minimum height for raised floor. This leads some people to the erroneous conclusion that data centers without raised floor are somehow less reliable. This interpretation is incorrect . Raised floor allows for different design and operational implementations but neither those implementations or the raised floor itself impart more reliability. In fact, in high seismic zones just the opposite IS TRUE - slab has a higher calculated reliability than raised floor.
Calculated reliability is where we need to go as an industry. We need to thoroughly analyze design and operating standards to identify single points of failure, sources of cascading failures, maintenance considerations and a range of other concerns that determine reliability. Where appropriate, we need to back these up with detailed and vallidated calculations.