robustness
A valid item keeps its key and its difficulty when nothing that matters changes. Three kinds of change are tested.
| Variant | Applies to | What changes | Key |
|---|---|---|---|
| Paraphrase | sample-0 item of each arm, reference instance 0 | stem wording; a deterministic check confirms every number is preserved | unchanged |
| Numeric | reference items | fresh numbers from the same template, context held fixed | recomputed exactly, re-verified |
| Entity | reference items with a surface context | people, groups, attribute, or measured quantity | unchanged by construction |