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Diagnoses for 1970-1978 are based on the International Classification of Diseases (ICD), Eighth Revision, Adapted (ICDA-8) (3); those for 1979-1986, on the ICD, Ninth Revision, Clinical Modification (ICD-9-CM) (4).
Use of trade names is for identification only and does not imply endorsement by the Public Health Service or the U.S. Department of Health and Human Services.
To assess the effects of changes in coding practices from ICDA-8 to ICD-9-CM, NCHS calculated comparability ratios for various diseases, including cerebrovascular diseases. The comparability ratio of a disease entity is defined as the ratio of the number of cases coded to a set of ICD codes under the old coding procedures to the number of cases coded to a set of codes (not necessarily identical) under the new coding procedures, when the coding procedures are applied to the same cases. To obtain its comparability ratios, NCHS recoded data from 1975 using ICD-9-CM procedures. Even the adjusted rates seem to be affected by changes in coding procedures from ICDA-8 to ICD-9-CM and therefore are not used in this report.