At the most basic level, platform costs may be compared in terms of dollars per terabyte ($/TB) of user data. This metric, however, deals only with acquisition prices. There are typically significant differences in data structures, compression levels and performance between platforms that affect its validity. A more realistic measurement is offered by cost of ownership. Calculations should allow for acquisition costs as well as maintenance and support, personnel costs for database and system administration, deployment costs – it may take weeks to months of work to bring systems into production – and facilities costs over multi-year periods.