Diagnosing a common rater halo effect using the polytomous Rasch model.

The 'halo effect' may be unique to different raters or common to all raters. When common to all raters, halo is not detectable through standard fit indices of the three-facet Rasch model used to account for differences in rater severities. Using a formulation of halo as a violation of local independence, a halo effect common to all raters is simulated and shown to be diagnosable through contrasts between two-facet stack and rack Rasch analyses. In the former, the thresholds are clustered and the distribution of persons is multimodal; in the latter, all thresholds are close together and the distribution of persons is unimodal. In the former, the scale is stretched, and the person separation inflated, relative to the latter.