Unlocking the numerator-denominator bias III: adjustment ratios by ethnicity for 1981-1999 mortality data. The New...

Maori and Pacific deaths are under-counted in mortality data relative to census data. This 'numerator-denominator' bias means that routinely calculated mortality rates by ethnicity are incorrect. We used New Zealand Census-Mortality Study data to quantify the bias from 1981 to 1999.

The 1981, 1986, 1991 and 1996 Censuses were each anonymously and probabilistically linked to three years of subsequent mortality data, allowing a comparison of ethnicity recording.

Compared with death registrations, 16% more 0-74 year old decedents during 1981-1984 had self-identified as '1/2 or more Maori' on the 1981 Census, and 32% more during both 1986-1989 and 1991-1994 had self-identified as 'sole Maori' on the 1986 and 1991 Censuses. From September 1995, mortality data have allowed multiple ethnicity to be recorded. During 1996-1999, 7% more decedents identified Maori as one of their ethnic groups on the 1996 Census compared with mortality data. For Pacific decedents, 55%, 76% and 68% more self-identified as 'sole Pacific' on census data compared with data recorded on death registrations for 1981-1984, 1986-1989 and 1991-1994 respectively, but there was no difference for 1996-1999. The bias for Maori (but not for Pacific) was greater among the young and those living in central and southern regions of New Zealand.

The 1995 change to ethnicity recording on mortality data has improved the robustness of ethnicity data collection. These adjustment factors for 1981-1999 allow for more accurate calculations of ethnic-specific mortality rates over the last 20 years.