Equivalence Classes cluster test input and test output data into classes where representative values of the same equivalence class indicate a similar behavior of a specific test.

Int. Organization for Standardization, "Int. Standard ISO/IEC/IEEE 29119-4:2015 Software and systems engineering - Software testing - Part 4: Testing techniques," 2015. [ISO Standard]

A very simple boiling point sensor for water, an Equivalence Class 'not boiling' can contain all values below 100°Celsius. The expected behavior for all values of this equivalence class are that the water does not boil. Equivalence Classes represent either valid values for the particular input parameter, e.g. 100°Celsius, or invalid values, e.g. -300°Celsius.

K. Meixner, D. Winkler, and S. Biffl. Supporting Domain Experts by using Model-Based Equivalence Class Partitioning for Efficient Test Data Generation. In 2019 IEEE 24th Conference on Emerging Technologies and Factory Automation (ETFA), 2019 [DOI]