The kappa coefficient can be used for scales with more than 2 categories. Richards et al12 assessed intraobserver and interobserver agreement of radiographic classification of scoliosis in relation to the King classification system. The King system is a multicategory nominal scale by means of which radiographs of the spine can be classified into 1 of 5 types of spinal curve. However, many multicategory scales are ordinal, and in such cases it is important to retain the hierarchical nature of the categories.