it applies to grey box testing as well. Imagine an interface to a component which has a valid range between 1 and 12 like the example above. However internally the function may have a differentiation of values between 1 and 6 and the values between 7 and 12. Depending upon the input value the software internally will run through different paths to perform slightly different actions. Regarding the input and output interfaces to the component this difference will not be noticed, however in your grey-box testing you would like to make sure that both paths are examined. To achieve this it is necessary to introduce additional equivalence partitions which would not be needed for black-box testing. For this example this would be: ... -2 -1 0 1 ..... 6 7 ..... 12 13 14 15 ..... --------------|---------|----------|--------------------- invalid partition 1 P1 P2 invalid partition 2 valid partitions To check for the expected results you would need to evaluate some internal intermediate values rather than the output interface. It is not necessary that we should use multiple values from each partition. In the above scenario we can take -2 from invalid partition 1, 6 from valid partition P1, 7 from valid partition P2 and 15 from invalid partition 2. Equivalence partitioning is not a stand-alone method to determine test cases. It has to be supplemented by boundary value analysis. Having determined the partitions of possible inputs the method of boundary value analysis has to be applied to select the most effective test cases out of these partitions. Limitations In cases where the data ranges or sets involved approach simplicity (Example: 0-10, 11-20, 21-30), and testing all values would be practical, blanket test coverage using all values within and bordering the ranges should be considered. Blanket test coverage can reveal bugs that would not be caught using the equivalence partitioning method, if the software includes sub-partitions which are unknown to the tester. Also, in simplistic cases, the benefit of reducing the number of test values by using equivalence partitioning is diminished, in comparison to cases involving larger ranges (Example: 0-1000, 1001-2000, 2001-3000). Further reading The Testing Standards Working Party website Parteg, a free test generation tool that is combining test path generation from UML state machines with equivalence class generation of input values. https://books.google.com/books/about/Software_Testing_Techniques.html References Software testing