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When considering ‘affective lability’ as a major component in borderline personality disorder (BPD), Kendler et al. (1) could recently demonstrate that neuroticism and extraversion displayed unidimensional structures, implying that also BPD exists on a severity spectrum with normality. When Eysenck identified neuroticism and extraversion as two ‘big’ factors in the description of normal personality functioning (2), he also made an attempt to identify a third factor, namely psychoticism, however, without success.

In this issue of Acta Psychiatrica Scandinavica, Ahmed et al. have now investigated to what extent psychotic experiences or reality-distortion symptoms exist on a severity spectrum with normality. In their study, only self-reported symptoms have been examined (e.g. hallucinations or delusional experiences). Psychotic symptoms observed by psychiatrics (e.g. conceptual disorganization, emotional withdrawal and blunted or inappropriate affect) have not been examined. However, compared to the items in Eysenck’s psychoticism questionnaire, the items covering the Ahmed et al. (3) universe of psychotic experiences seem to possess a much higher clinical validity.

In the psychometric validation procedure, Ahmed et al. (3) used rather sophisticated models with overlap to factor analysis, namely different taxometric methods. The results tended to support a dimensional approach, implying that psychotic experiences might be another important factor in the description of normal functioning.

Whereas neuroticism and extraversion have stood the test of time over many years, the dimension of psychotic experiences still needs to be confirmed by empirical studies. However, the clinical validity of the dimension on which antipsychotic medication acts has been found high when using the clinician-administered Brief Psychiatric Rating Scale (BPRS) or the corresponding Positive and Negative Syndrome Scale (4). Within the BPRS, we have patient-reported symptoms of psychotic experiences (hallucinations and delusions) similar to the items used by Ahmed et al. (3). However, in the BPRS, we also have the specific schizophrenia symptoms to be observed by the psychiatrists (conceptual disorganization, emotional withdrawal and blunted affect). These self-reported items of psychotic experiences and the more specific schizophrenia items taken together have been found to be unidimensional when tested by item response theory models (4). These models are developed to test hypotheses about clinically valid dimensions, i.e., the invariant item ordering, by demonstrating that items with low prevalence have to be preceded by items with higher prevalence. This invariant item ordering is a measurement requirement, indicating that the summed item score is a sufficient statistic (4, 5). There still seems to be too much overlap between the taxometric methods and the family of factor analysis. Thus, an examination of eigenvalues within the taxometric methods only provides a very rough check of the existence of unidimensionality.

In conclusion, this very first attempt to see a dimensional structure for psychotic experiences through taxometric glasses should stimulate research into the association between severity scores and treatment. Psychometrically, it is a matter of standardization, i.e., to identify the severity cut-off score at which treatment is needed, as well as the cut-off score at which remission is obtained (the very goal of treatment). In this standardization procedure, we do need to implement the response theory models to be sure that the total sum score of psychotic experiences is a sufficient statistic (4, 5).

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