_Mdhad_ _3_ data were reported. Diabetes, nephritis, cirrhosis of the liver, and ulcers of the stomach and duodenum have been included among circulatory system diseases not because of any presumed simaty in etiology or pathogenesis but to keep the number of categories manageable by combining the degenerative diseases other than cancer.

Hakulinen and others also included in their analysis two subcategories for infectious and parasitic diseases: diarhea, gastritis, and enteritis; and influenza, pneumonia, and bronchitis. Only the diarrhea subcategory is maintained as a specific cause. The second subcategory is further refined into infectious and chronic components. Other subcategories for specific causes of death (Table 1) were chosen to break up the three largest of the major categories-infectious and parasitic diseases, circulatory system diseases, and other causes. Subcategories were chosen mainly for substantive interest. The diarrhea subcategory is duplicated among these specific causes, but the influenza, pneumonia, and bronchitis subcategory is replaced by otheis somewhat more specific. These breakdowns also allow distinction of the specific causes just noted that fit awkwardly into the major categories.

Among the specific causes, not all could be defined witS precision. For micronutrient disorders, for instance, the codes selected from the revision currently in use (the ninth) of the International Statistical Classification of Diseases, Injuries, and Causes of Death (ICII-9; VHO 1977) were thfose for disorders of hie thyroid gland, aneemias, and avitamntosis. This group of causes may be both too broad and too narrow in different ways, but further specification was not possible.

# _Data on causes of death_

Statistics on causes of death, by sex and age, based on the International Class-5cation of Diseases are provided annually to WHO by some 70 countries or territories. In principle, the cause of death returns should be comparable between countries since they are based on a common set of procedures for coding and certifying the cause of death. In practice, however, the comparability of data across countries is affected by variations in diagnostic preferences, cultural factors, medical training, the availability of diagnostic aids, and other factors.

In addition, the reliability of national cause of death statistics is a matter of concern. Broadly speaking, reliability depends on coverage and quality. Deaths not covered by the vital registration system are most likely those in more remote areas with a cause of death structure typical of high mortality populations. Thus countries with incomplete coverage are likely to report a cause of death structure biased towards the chronic diseases pattem more commonly found in urban areas; the greater the incompleteness, the more biased the data are likely to be. However, of even greater significance with respect to reliability is the extent of medical certification of the cause of death, which ensures data qualty. In remote areas with few if any physicians, cause of death is frequently certified by lay personnel who have little or no medical training. Diagnosis is uncertain and many deaths may be coded to symptoms and ill-defined conditions.