Reliability and factorial structure of a rating scale for persistent vegetative state

We developed a new rating scale, Kohnan Vegetative Score, to measure severity and small clinical changes in vegetative state patients. It has 7 items corresponding to the conditions of vegetative state by Japanese Society of Neurosurgery: motor function, food ingestion, urination and defecation, eye movement, vocalization, communication, and facial expression. Each item is rated in 5 ordinal categories: slight (score=l), mild(2), moderate(4), and extreme(5). The sum of the scores is used as the summary score, which ranges from 7 to 35, and high score means 'severe'. We examined the reliability and the factorial structure of the Kohnan Vegetative Score. The subjects were 10 patients who met the conditions of vegetative state. Four neurosurgeons rated the subjects, and then 2 of them repeated the rating after one week interval. As a measure of reliability, the (weighted) Kappa coefficient proposed Cohen (1960, 1968) was calculated for each item, and the intraclass correlation coefficient (ICC) was calculated for the summary score. To analyze the factorial structure, the factor analysis was carried out. The minimum and the maximum weighted Kappa values were 0.44 and 0.64 for intra-rater reliability, and 0.37 and 0.69 for inter-rater reliability, respectively. Concerning the factorial structure, the contribution of the first factor was 91.5% which indicated the unidimensionality of the scale. The ICC's estimate for the summary score were 0.90 (95% C.I.: 0.766-0.970). On the basis of these results, the Kohnan Vegetative Score has unidimensionality and high reliability enough for a practical use.