In drug development research, developing a “good” measure of disease change is important. A major issue in design and analysis of rheumatoid arthritis (RA) studies is defining an endpoint that reflects multidimensional progression of the disease. The researchers of RA use a set of measures -- joint counts, pain assessment, physician and patient global assessments, functional disability and acute phase reactants-- to evaluate the patient disease activity. ACR20 (American College of Rheumatology 20% improvement criteria), which is currently accepted as a composite measure of treatment effectiveness by the rheumatologic community, is a binary index. In this presentation, we will discuss the problem related with ACR20. Some results from the validation of ‘CDAI-Lite’ that some leading researchers propose as a new measure of disease activity, will be presented and discussed.