A system is an organized set of related components established to accomplish a certain task. A computer system has a computer as one of its components. A systems analysis, a study of an existing system, to determine both how it works and how well it meets the needs. A users, who are usually employees and customers. A systems design, the development of a plan for an improved system. A systems analyst normally does both the analysis and design. Some people do both programming and analysis . The systems development life cycle (SDLC) can be described in five phases: (1) preliminary investigation, (2) analysis, (3) design, (4) development, and (5) implementation. Phase 1, preliminary investigation, also known as the feasibility study or system survey, is the initial consideration of the problem to determine how--and whether--an analysis and design project should proceed. After determining the nature of the problem and its scope, the analyst expresses the users' needs as objectives. In phase 2, systems analysis, the analyst gathers and analyzes data from common sources such as written documents, interviews, questionnaires, observation, and sampling. A data flow diagram (DFD) provides an easy-to-follow picture of the flow of data through the system. A system requirements, a detailed list of the things the system must be able to do. In phase 3, systems design, the analyst submits a general preliminary design for the client's approval before proceeding to the specific detail design. Preliminary design begins with reviewing the system requirements. A prototype, a limited working system or part of a system that gives users a preview of how the new system will work. Detail design normally involves considering the parts of the system in the following order: output requirements, input requirements, files and databases, systems processing, and systems controls and backup Phase 4, systems development, consists of scheduling, programming, and testing. Schedule deadlines and milestones . Phase 5, implementation, includes training to prepare users of the new system. System conversion may be done in one of four ways: direct conversion, immediately replacing the old system with the new system; phased conversion, easing in the new system a step at a time; pilot conversion, testing the entire system with a few users and extending it to the rest when it proves successful; and parallel conversion, operating the old and new systems concurrently until the new system is proved successful.