A USFHS official adaptation of a system for classifying diseases and operations for the purpose of indexing hospital records developed by the World Health Organization. Diseases are grouped according to the problems they present. For example, the major infective and parasitic diseases are listed in one section and all malignant neoplasms in another section. A three digit numerical code is used to number the major disease categories. Subdivisions within categories are numbered with a four digit code. The ICDA is revised every ten years. The eighth version (known as ICDA-8) is now in use and ICDA-9 in preparation. A further revised and expanded version of ICDA-8, known as Hospital-ICDA, was developed by CHPA and is widely used in hospitals and the Professional Activity Study. This version is now in its second edition, H-ICDA-2.