According to the General Electric's Model of 1977 by Jim McCall, quality is defined by three areas: product revision, product transition, and product operation. Product revision focuses on a product's ability to undergo changes through maintainability, flexibility, and testability. Product transition centers on a product's adaptability through new environments via portability, reusability, and interoperability. Product operation examines operation characteristics such as correctness, reliability, efficiency, integrity, and usability.