The analysts who have developed the functional specifications and specified the design of the system usually perform system testing. System testing is often termed black box testing because the testing is concerned with proving that certain inputs produce predicted outputs. It is not concerned with proving the internal logic of the programs that cause the transformation to take place. This has already been done in unit testing. Hence system testing is concerned with ensuring that the system meets the functional requirements defined in the requirements specification. It also considers the general usability of the software. In some organizations system testing includes load or stress testing which tests the system under operational conditions, ensuring it can still deliver the required performance when handling operations volumes by the agreed number of users.