Evaluating the comparability of different grouping schemes for mortality and morbidity

Comparison of mortality and morbidity is a commonly used method in health related studies. The International Classification of Disease (ICD) consists of thousands of codes for classifying cause of death and disease categories. A grouping scheme is needed to cluster related categories into a meaningful and manageable number for comparative purposes. Different kinds of grouping schemes have been used; nevertheless, little is known about the comparability among different grouping schemes. In this study, we compared seven grouping schemes; five for mortality and two for morbidity. We found poor comparability between different grouping schemes. Different schemes covered different ranges of codes. Some schemes used the same title, but included different ranges of codes. Features of newly developed grouping schemes were to group disease categories of similar characteristics across traditional ICD chapters and to group disease categories based on health care needs, instead of those based merely on etiology or organ system. Different grouping schemes were developed to reveal the unique mortality and morbidity pattern of different geographical areas. Different grouping logic was used by different grouping schemes. Therefore, it is difficult to make a good comparison between different schemes. An investigator tabulating the mortality or morbidity figures based on a given grouping scheme should explicitly define the exact ICD codes included. Any user of data derived from different grouping schemes, especially for comparisons between countries, should be cautious about the comparability problems.