Measures of individual and group changes in ordered categorical data: application to the ADL staircase.

The aim was to describe the application of a non-parametric method to a study of changes in activities of daily living (ADL) measured by a 10-level ordinal scale, the Staircase of ADL, in a population of 70-year-old persons followed to the age of 76 (n = 371). The statistical method takes account of the fact that change cannot be defined by differences and can separately measure the level of change for the group and for the individuals. This is demonstrated step by step. Measures of change for the subgroups, men and women, are also given. As a group, there was a systematic increase in ADL dependence between the ages of 70 and 76 years. The individual pattern of change was more dispersed between 73 and 76 years of age. Individual variations in ADL changes with increasing age were small compared to the systematic change for ADL for the subgroups of men and women. The systematic change in ADL follows a pattern with "one step at a time" for women and "many steps at a time" for men. In conclusion, this new statistical method provides a valuable tool for detailed information of individual as well as group changes in longitudinal studies.