Hypnotizability Norms may not be Representative of the General Population: Potential Sample and Self-Selection Bias Considerations.

The analysis of the methods sections of 66 normalization tests of hypnotizability scales reveals that out of 33,338 subjects, 58.57% were college and university students, and the majority of these were students of psychology. Of all subjects, 7.45% were younger school children, 27.63% were patients treated with hypnosis, and out of these, 85.26% were patients of 1 single therapist. Only 0.51% were trainees of dental or nursing schools, 0.13% were prisoners, and 5.71% were other adults. These figures suggest a sample-selection bias. As 83.08% of these subjects were told beforehand that they were to undergo a hypnosis study, a self-selection bias is also implied in the data. It can be presumed that those interested in hypnosis participated, whereas others who had no interest in hypnosis may have refrained. It is concluded that some of the published norms of hypnotizability tests may not be adequately representative of the general population. Many hypnosis studies, whether clinical or experimental, which are based on hypnotizability, may be afflicted by these biases.