To understand trends in cause-specific mortality spanning the change from ICD-9 to ICD-10, we need to measure the effect of this change on the proportion of deaths attributed to different causes of death. This is done using bridge coding, that is coding a sample of death certificates independently to both ICD-9 and ICD-10, and comparing the resulting underlying causes of death. The first step in this process is to identify equivalent codes or code groups in the two revisions which represent the same causes. In most cases this is not contentious, and the same groupings have been used by various authors and national statistics offices.
The results are then presented as comparability ratios of the numbers of deaths assigned to a given disease or group of diseases in the two revisions. These are simply the ratio of the number of deaths coded to a cause in ICD-10 to the number coded to the equivalent cause in ICD-9. These comparability ratios can then be used to adjust comparisons of cause-specific mortality rates at or near the time of the changeover.
Articles containing more detailed information are available in Health Statistics Quarterly numbers 8, 13 and 14.