Typically we have always looked at the levels of testing like unit, integration and system from the aspect of the “size” of entity-under test. Unit test is typically understood as being done on the smallest component that can be independently tested. Integration test is typically understood as being done once the various units have been integrated. System test is typically seen as the last stage of validation and is done on the whole system.
What specific types of defects are expected to be uncovered at each of these test levels is typically not clear. This lack of clarity results in test cases being insufficient or redundant. In HyBIST, the focus shifts to specific types of defects to be detected, and the act of detection is staged to ensure an efficient detection approach.
In HyBIST, the notion is of quality levels, where each quality level represents a milestone towards meeting the final cleanliness criteria. At each level, the focus is on uncovering certain types of defects and therefore there is a sharp clarity of the objective of what we are looking for at each level. Each quality level represents a step in the staircase of quality.