ETS researcher Martha Stocking has quipped that most adaptive tests are actually barely adaptive tests (BAT's) because, in practice, many constraints are imposed upon item choice. For example, CAT exams must usually meet content specifications; a verbal exam may need to be composed of equal numbers of analogies, fill-in-the-blank and synonym item types. Also, on some tests, an attempt is made to balance surface characteristics of the items such as gender of the people in the items or the ethnicities implied by their names. Thus CAT exams are frequently constrained in which items it may choose and for some exams the constraints may be substantial and require complex search strategies (e.g., linear programming) to find suitable items.
Wim van der Linden and his coauthors[9] have advanced an alternative approach called shadow testing which involves creating entire shadow tests as part of selecting items. Selecting items from shadow tests helps adaptive tests meet selection criteria by focusing on globally optimal choices (as opposed to choices that are optimal for a given item).