If more is known about the proximities or the objects, then additional restrictions (or constraints) can be imposed on the MDS model. This usually means that the MDS solutions must satisfy certain additional properties of the points’ coordinates or the distances. These properties are derived from substantive considerations. The advantage of enforcing such additional properties onto the MDS model is that one thus gets direct feedback about the validity of one’s theory about the data. If the Stress of a confirmatory MDS solution is not much higher than the Stress of a standard (“unconstrained”) MDS solution, the former is accepted as an adequate model. Several procedures that allow one to impose such external constraints are described and illustrated.
KeywordsExternal Variable External Constraint Side Constraint External Scale Optimal Transformation
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