"Three notions of item equivalency are distinguished. They correspond to the back-translation approach, the psychometric IRT approach, and the facet-theoretical approach. The latter defines equivalent item as items that answer the same questions. The question, then, is explicated in terms of its design. This yields the item's blueprint. One can extract such blueprints by studying given items, but the result is generally not unique. Nevertheless, it makes it possible to predict empirical regularities for the items and, therefore, tests for equivalency. If the tests fail, however, item non-equivalency is just one possible explanation. Design-equivalency is, on the other hand, a definitional issue, not an empirical one. The enmpirical issue is the design's usefulness for a particular purpose, usually for answering the research question." (author's abstract
