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

To overcome problems with previous psychometric approaches to hypnosis scales, the authors applied full-information 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 HGSHS:A 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.