Mortality by ethnic group to 2006: is extending census-mortality linkage robust?

Objective: To update trends in mortality by ethnic group from the New Zealand Census-Mortality Study (NZCMS), by additionally linking 2004-06 mortality records to the 2001 Census. To investigate possible bias from this extended linkage, especially for Pacific and Asian people who emigrate more frequently.

Methods: Anonymous and probabilistic record linkage of 2004-06 mortality records with the 2001 Census was undertaken. Age-standardised 1-74 year old mortality rates by sex and age group, and for all-cause and selected causes of death, were calculated using the direct method for first 30 months post 2001 Census (2001-03) and second 30 months (2003-06).

Results: Observed all-cause mortality rates continued to fall in 2003-06 compared to previous periods, but more so for Pacific (18.3% and 21.7% for males and females for 2003-06 compared to 2001-04, respectively) and Asian (22.2%, 16.7%), than for Maori (13.2%, 14.2%) and European/Other (13.0%, 10.4%). Observed rate ratios for Maori, compared to European/Other were 2.43 (95% CI 2.31-2.57) for males and 2.72 (2.56-2.89) for females, the same (males) and slightly less (7%, females) than in 2001-03. Declines in cardiovascular disease (CVD) and injury mortality were the main drivers of all-cause mortality rate reductions for all ethnic groups. Relative inequalities in CVD between Maori and European/Other remain high (three to four-fold relative risks), but reduced by 8% for both males and females from 2001-03 to 2003-06, which in turn means that absolute inequalities closed by as much as 20%.

Conclusion: We suspect that analyses comparing mortality rates over time within one of the closed NZCMS cohorts (e.g. 2001-03 compared to 2003-06) is prone to bias due to our inability to censor people when they migrate out of New Zealand. This limitation means mortality rates in the NZCMS are increasingly underestimated with time since census night, particularly for Pacific and Asian people. However, previously published NZCMS trends remain valid as the duration of follow-up (3 years) is short, and cohorts were not split by time since census. Nevertheless, it is safe to conclude that mortality rates continued to decline from 2001-03 to 2003-04 for all four ethnic groups. All-cause mortality inequalities for Maori compared to European/Other over this time were probably stable in relative terms and decreasing in absolute terms, but cardiovascular disease (CVD) inequalities probably decreased in both absolute and relative terms.