Dr. North Replies

To the Editor: Drs. Asmundson and Taylor and Drs. Pietrzak and Southwick suggest the importance of four-factor analysis models toward validation and revision of PTSD criteria. Although factor analysis may provide a worthwhile test of the natural co-occurrence of symptoms to define cohesive categories whose members tend to travel together, it is a starting point to establish alternative models of symptom clusters, which will then need to be subjected to validity testing. Establishment of a model through factor analysis is not a diagnostic validation procedure. Validity of a proposed set of diagnostic criteria is determined through established validation methods in the following five phases: examination of cohesive core characteristics, laboratory studies, exclusion criteria, follow-up studies, and family studies (1) . Factor analysis methods may be contributory to the first of these validation phases. The models that perform best in validation studies would be potential candidates for consideration of revision of PTSD criteria in DSM-V. We would discourage revisions to DSM-V PTSD criteria based on factor analysis before the models that emerge from this research are subjected to validation procedures.