Which is considered a random cause that cannot be identified?

Assignable causes of variation are identifiable sources that can be tracked and attributed to specific factors affecting a process. These causes can often be eliminated or controlled through adjustments, making them distinct from random causes that cannot be singled out.

Structural variation refers to variations that arise from changes in the system's physical setup or design. These changes can often be identified and addressed, thus they do not fit the definition of a random cause. Unlike common causes, structural variations can be controlled and modified to improve process performance.

Tampering involves making unnecessary adjustments or interventions in a process based on perceived problems, which can introduce additional variation. While it is an identifiable issue, it represents a deliberate action rather than a random, untraceable cause of variation.