In fairness to the FFM, it should perhaps be acknowledged that a potential limitation of the current and prior meta-analyses is that the personality disorder symptomatology has been confined to the existing DSM-IV-TR nomenclature (APA, 2000). This could be problematic, as the nomenclature might not provide full coverage of maladaptive personality functioning. The personality disorder section of the diagnostic manual has not been entirely stable or settled, with diagnoses being added or deleted with each revision. Additionally, clinicians provide a diagnosis of not otherwise specified (NOS) when they determine that a person has that particular class of mental disorder but whose symptoms are not adequately represented by any of the ten diagnostic categories (APA, 2000). Personality disorder NOS is often the single most frequently used diagnosis in clinical practice, as indicated in studies of clinical records and in a meta-analysis of NOS usage across structured and unstructured assessments (Verheul & Widiger, 2004). It is not entirely clear how clinicians are using PDNOS within their practice, but many of these studies suggest that clinicians do not find the existing categories adequate in covering personality disorder symptomatology (Verheul & Widiger, 2004; Westen & Arkowitz-Westen, 1998).
