When an instrument is designed to produce measures of more than one attribute, interactions with medical conditions may obscure the distinctions between the measures.
Items are one word or a brief phrase that describe a symptom of depression.
Data were collected from 433 multiple sclerosis patients. All items were combined into one Rasch analysis, followed by a principal component analysis of the standardized residuals. The utility of the instrument for MS patients would be supported if the factor structure reflected the instrument's intended functioning. Figure 1 shows that the first factor in the residuals contrasts the 9 Vegetative items against the others. After removal of the Vegetative items from the analysis, the first factor contrasts, in Figure 2, the Evaluative against the Mood items.