Software Cost reduction (SCR) is a set of techniques for designing software systems developed by David Parnas and researchers from the U.S. Naval Research Laboratory (NRL) beginning in the late 1970s. A major goal of the original SCR research team was to evaluate the utility and scalability of software engineering principles by applying the principles to the reconstruction of software for a practical system, the Operational Flight Program (OFP) for the U.S. Navy's A-7 aircraft. The process of applying the principles produced a number of new techniques for software design, which were demonstrated in a requirements document and several software design documents for the A-7. Further research during the 1990s produced two formal models, the Four Variable Model and the SCR requirements model, and a set of software tools for analyzing SCR-style requirements documents.

A central notion of SCR is that software should be designed using an idealized process called the “rational design process”. Although designing software using a perfectly rational process is impossible, software developers are more likely to produce a rational design if they follow a rational process rather than if they proceed on an ad hoc basis. In the rational design process, software is designed and implemented in stages. At each stage, a work product, such as a requirements document or a design document, is produced. Each work product is associated with criteria that the work product must satisfy and a description of the information that the work product contains. This article focuses on the SCR techniques for constructing and evaluating the requirements document, the work product built during the requirements stage of software development, and the aspect of SCR that has received significant attention during both the early and the more recent research. It also briefly describes, and gives pointers to, the SCR approach to software design, focusing on the design and documentation of the module structure, the module interfaces, and the uses hierarchy.