This specifies which facets are to be centered. Centered facets are constrained to have a mean element measure of zero. In most analyses, if more than one facet is non-centered, the frame of reference is not sufficiently constrained and ambiguity results. The usual convention is to center the frame of reference on the agents of measurement: items, tasks, judges. This establishes the origin. Then the objects of measurement, persons, are positioned with respect to this origin.

 

If both Noncenter= and Center= are specified, only the first encountered is actioned.

 

When any element within a facet is anchored, or group anchored, then the origin of the scale for that facet is set by the anchoring, and that facet is automatically non-centered.

 

Example 1: Items, facet 2 and raters, facets, are to be centered, then

Center=2, 3

 

Example 2: No facets are to be centered. This is an unusual situation such as when there is only one active facet so that the item difficulties are defined relative to an implicit origin, as in the "Three Mile" example.

Center=0