Regression testability refers to the property of a program, modification, or test suite that lets it be effective and efficiently regression-tested. Leung and White classify a program as regression testable if most single statement modifications to the program entail rerunning a small proportion of the current test suite. Under this definition, regression testability is a function of both the design of the program and the test suite.
To consider regression testability, a regression number is computed. It is the average number of affected test cases in the test suite that are affected by any modification to a single instruction. This number is computed using information about the test suite coverage of the program.
If regression testability is considered at an early stage of development, it can provide significant savings in the cost of development and maintenance of the software.