This International Standard defines how user oriented performance of computer-based software systems (CBSS) may be measured and rated. A CBSS is a data processing system as it is seen by its users, e.g. by users at various terminals, or as it is seen by operational users and business users at the data processing center. A CBSS includes hardware and all its software (system software and application software) which is needed to realize the data processing functions required by the users or what may influence to the CBSS’s time behaviour. This International Standard is applicable for tests of all time constrained systems or system parts. Also a network may be part of a system or may be the main subject of a test. The method defined in this International Standard is not limited to special cases like classic batch or terminal-host systems, e.g. also included are client server systems or, with a broader comprehension of the definition of ‘task’, real time systems. But the practicability of tests may be limited by the expenditure required to test large environments. This International Standard specifies the key figures of user oriented performance terms and specifies a method of measuring and rating these performance values. The specified performance values are those which describe the execution speed of user orders (tasks), namely the triple of: -- execution time, -- throughput, -- timeliness. The user orders, subsequently called tasks, may be of simple or complex internal structure. A task may be a job, transaction, process or a more complex structure, but with a defined start and end depending on the needs of the evaluator. When evaluating the performance it is possible to use this International Standard for measuring the time behaviour with reference to business transaction completion times in addition to other individual response times.