Comparing traditional and Rasch analyses of the Mississippi PTSD Scale: revealing limitations of reverse-scored items.

This study examined whether Rasch analysis could provide more information than true score theory (TST) in determining the usefulness of reverse-scored items in the Mississippi Scale for Posttraumatic Stress Disorder (M-PTSD). Subjects were 803 individuals in inpatient PTSD units at 10 VA sites. TST indicated that the M-PTSD performed well and could be improved slightly by deleting one item. Factor analysis using raw scores indicated that the reverse-scored items formed the second factor and had poor relationships with normally scored items. However, since item-total correlations supported their usefulness, they were kept. The subsequent Rasch analysis indicated that five of the seven worst fitting items were reverse-scored items. We concluded that using reversed items with disturbed patients can cause confusion that reduces reliability. Deleting them improved validity without loss of reliability. The study supports the use of Rasch analysis over TST in health research since it indicated ways to reduce respondent burden while maintaining reliability and improving validity.