Comparability ratio

About every 10–20 years, the International Classification of Diseases (ICD) is revised to stay abreast of advances in medical science and changes in medical terminology. Each of these revisions produces breaks in the continuity of cause-of-death statistics because of changes in classification and in the rules for selecting an underlying cause of death. Classification and rule changes affect cause-of-death trend data by shifting deaths away from some cause-of-death categories and into others. Comparability ratios measure the effect of changes in classification and coding rules. For the causes shown in CompRatio-Table, comparability ratios range between 0.6974 and 1.5812. Influenza and pneumonia had the lowest comparability ratio (0.6974), indicating that this cause is about 30% less likely to be selected as the underlying cause of death under the 10th revision of ICD (ICD–10) than under the 9th revision (ICD–9). Alzheimer’s disease had the highest comparability ratio (1.5812), indicating that Alzheimer’s disease is 58% more likely to be selected as the underlying cause when ICD–10 coding is used.

For selected causes of death, the ICD–9 codes used to calculate death rates for 1980–1998 differ from the ICD–9 codes most nearly comparable with the corresponding ICD–10 cause-of-death category, which also affects the ability to compare death rates across ICD revisions. Examples of these causes are Ischemic heart disease; Cerebrovascular diseases; Trachea, bronchus, and lung cancer; Unintentional injuries; and Homicide. To address this source of discontinuity, mortality trends for 1980–1998 were recalculated using ICD–9 codes that are more comparable with codes for corresponding ICD–10 categories. See Sources and Definitions, Cause of death for the ICD–9 codes used for these causes. This modification may lessen the discontinuity between the 9th and 10th revisions, but the effect on the discontinuity between the 8th and 9th revisions is not measured.

Comparability ratios shown in the CompRatio-Table are based on a comparability study in which the same deaths were coded using both the 9th and 10th revisions. The comparability ratio was calculated by dividing the number of deaths classified using ICD–10 by the number of deaths classified using ICD–9. The resulting ratios represent the net effect of the 10th revision on cause-of-death statistics and can be used to adjust mortality statistics for causes of death classified by the 9th revision to be comparable with cause-specific mortality statistics classified by the 10th revision.

The application of comparability ratios to mortality statistics helps make the analysis of change between 1998 and 1999 more accurate and complete. The 1998 comparability-modified death rate is calculated by multiplying the comparability ratio by the 1998 death rate. Comparability-modified rates should be used to estimate mortality change between 1998 and 1999.

Applying the comparability ratios presented in the CompRatio-Table to age-, race-, and sex-specific mortality data may not be appropriate. Demographic subgroups may differ in their cause-of-death distribution, and this would result in demographic variation in cause-specific comparability ratios.

For more information, see: Anderson RN, Miniño AM, Hoyert DL, Rosenberg HM. Comparability of cause of death between ICD–9 and ICD–10: Preliminary estimates. National Vital Statistics Reports; vol 49 no 2. Hyattsville, MD: National Center for Health Statistics. 2001. Available from: https://www.cdc.gov/nchs/data/nvsr/nvsr49/nvsr49_02.pdf; Kochanek KD, Smith BL, Anderson RN. Deaths: Preliminary data for 1999. National Vital Statistics Reports; vol 49 no 3. Hyattsville, MD: National Center for Health Statistics. 2001. Available from: https://www.cdc.gov/nchs/data/nvsr/nvsr49/nvsr49_03.pdf and the ICD comparability ratio website at: https://www.cdc.gov/nchs/nvss/mortality/comparability_icd.htm. (Also see Sources and Definitions, Cause of death; International Classification of Diseases [ICD].)

CompRatio-Table. Comparability of selected causes of death between 9th and 10th revisions of International Classification of Diseases (ICD)

¹ See COD-Table in this report for ICD–9 and ICD–10 cause-of-death codes.

² Ratio of number of deaths classified by ICD–10 to number of deaths classified by ICD–9.

SOURCE: National Center for Health Statistics. Table 1. Final and preliminary comparability ratios for 113 selected causes of death. Available from: https://ftp.cdc.gov/pub/Health_Statistics/NCHS/Datasets/Comparability/icd9_icd10/Comparability_Ratio_tables.xls; and Miniño AM, Anderson RN, Fingerhut LA, Boudreault MA, Warner M. Deaths: Injuries, 2002. National Vital Statistics Reports; vol 54 no 10. Hyattsville, MD: National Center for Health Statistics. 2006. Available from: https://www.cdc.gov/nchs/data/nvsr/nvsr54/nvsr54_10.pdf.