If you like simplicity when it comes to considering which data center availability “standard” you want to follow, the 1-2-3-4 rating system may be just the right approach for you. However, these fixed steps restrict ways to consider variations in availability schemes, such as increasing the redundancy of the power chain to 2(N+1), while keeping the cooling system at N+1. Therefore, the 4-level system does not acknowledge that as the proverbial (but forbidden) 3+ rating. Unfortunately, this provides little or no flexibility for determining how well this reflects the interrelationship of your computing architecture with the facility infrastructure requirements.
Then consider taking a more holistic approach — The Green Grid Open has recently released the Open Standard for Data Center Availability (OSDA) in the form of an online tool to allow data center owners, operators, and users to project the overall availability of a data center. The OSDA project began in 2016, however this year I had the opportunity to be involved with part of the online developments. While I did not do the “heavy lifting” of applying the Monte Carlo Simulations, I can share that the workgroup put a lot of thought into, and many, many hours discussing and refining, the OSDA tool.