Four decades of group hypnosis scales: What does item-response theory tell us about what we've been measuring?

American Journal of Clinical Hypnosis,  Oct 2004  

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Sadler, P. & Woody, E. Z. (2004). Four decades of group hypnosis scales: What does item-response theory tell us about what we've been measuring? International Journal of Clinical & Experimental Hypnosis, 52, 132-158. To overcome problems with previous psychometric approaches to hypnosis scales, the authors applied fullinformation factor analysis, based on multidimensional item-response theory (IRT), to a 39-year sample of 11,517 records of the Harvard Group Scale of Hypnotic Susceptibility, Form A (HGSHS:A).

They also performed a comparable analysis on the standardization sample of the Waterloo-Stanford Group C Scale (WSGC). The HGSHS: A emerges as two-factored, whereas the WSGC more closely approaches unidimensionality. The HGSHS:A factor structure and means show very little change over 4 decades. However, IRT-based item analysis on the HGSHSiA indicates that problems such as "pseudoguessing" on 2 items limit the quality of the item set. The authors propose alternative substantive interpretations of the traits that may underlie the two-factor structure.

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