Iterative and incremental development is any combination of both iterative design (or iterative method) and incremental build model for development. Usage of the term began in software development, with a long-standing combination of the two terms iterative and incremental having been widely suggested for large development efforts. For example, the 1985 DOD-STD-2167 mentions (in section 4.1.2): "During software development, more than one iteration of the software development cycle may be in progress at the same time." and "This process may be described as an 'evolutionary acquisition' or 'incremental build' approach." In software, the relationship between iterations and increments is determined by the overall software development process.
Overview The basic idea behind this method is to develop a system through repeated cycles (iterative) and in smaller portions at a time (incremental), allowing software developers to take advantage of what was learned during development of earlier parts or versions of the system. Learning comes from both the development and use of the system, where possible key steps in the process start with a simple implementation of a subset of the software requirements and iteratively enhance the evolving versions until the full system is implemented. At each iteration, design modifications are made and new functional capabilities are added. The procedure itself consists of the initialization step, the iteration step, and the Project Control List. The initialization step creates a base version of the system. The goal for this initial implementation is to create a product to which the user can react. It should offer a sampling of the key aspects of the problem and provide a solution that is simple enough to understand and implement easily. To guide the iteration process, a project control list is created that contains a record of all tasks that need to be performed. It includes items such as new features to be implemented and areas of redesign of the existing solution. The control list is constantly being revised as a result of the analysis phase. An iteration involves redesign and implementation, which is meant to be simple, straightforward, and modular, supporting redesign at that stage or as a future task added to the project control list. The level of design detail is not dictated by the iterative approach. In a light-weight iterative project the code may represent the major source of documentation of the system; however, in a critical iterative project a formal Software Design Document may be used. The analysis of an iteration is based upon user feedback and the program analysis facilities available. It involves analysis of the structure, modularity, usability, reliability, efficiency, and achievement of goals. The project control list is modified in light of the analysis results.
Phases Incremental development slices the system functionality into increments (portions). In each increment, a slice of functionality is delivered through cross-discipline work, from the requirements to the deployment. The Unified Process groups increments/iterations into phases: inception, elaboration, construction, and transition.
Inception identifies project scope, requirements (functional and non-functional) and risks at a high level but in enough detail that work can be estimated. Elaboration delivers a working architecture that mitigates the top risks and fulfills the non-functional requirements. Construction incrementally fills-in the architecture with production-ready code produced from analysis, design, implementation, and testing of the functional requirements. Transition delivers the system into the production operating environment. Each of the phases may be divided into 1 or more iterations, which are usually time-boxed rather than feature-boxed. Architects and analysts work one iteration ahead of developers and testers to keep their work-product backlog full.
Usage and history Many examples of early usage are provided in Craig Larman and Victor Basili's article "Iterative and Incremental Development: A Brief History", with one of the earliest being NASA's 1960s Project Mercury. Some of those Mercury engineers later formed a new division within IBM, where "another early and striking example of a major IID success [was] the very heart of NASA’s space shuttle software—the primary avionics software system, which [they] built from 1977 to 1980. The team applied IID in a series of 17 iterations over 31 months, averaging around eight weeks per iteration. Their motivation for avoiding the waterfall life cycle was that the shuttle program’s requirements changed during the software development process." Some organizations, such as the US Department of Defense, have a preference for iterative methodologies, starting with MIL-STD-498 "clearly encouraging evolutionary acquisition and IID".
The DoD Instruction 5000.2 released in 2000 stated a clear preference for IID: There are two approaches, evolutionary and single step [waterfall], to full capability. An evolutionary approach is preferred. … [In this] approach, the ultimate capability delivered to the user is divided into two or more blocks, with increasing increments of capability...software development shall follow an iterative spiral development process in which continually expanding software versions are based on learning from earlier development. It can also be done in phases. Recent revisions to DoDI 5000.02 no longer refer to "spiral development," but do advocate the general approach as a baseline for software-intensive development/procurement programs. In addition, the United States Agency for International Development (USAID) also employs an iterative and incremental developmental approach to its programming cycle to design, monitor, evaluate, learn and adapt international development projects with a project management approach that focuses on incorporating collaboration, learning, and adaptation strategies to iterate and adapt programming.
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