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| Journal of the New Zealand Medical Association,
29-June-2012, Vol 125 No 1357
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Lavinia Tan, Tony A Blakely
The measurement and monitoring of ethnic inequalities in
health have long been of interest in New Zealand, both for research and policy.
There are large differences in mortality between ethnicities in New
Zealand.1–4
Māori and Pacific have higher mortality rates than
non-Māori non-Pacific non-Asian (nonMPA), while mortality rates of Asian
people (people from East, South East and South Asia, but excluding those from
the Middle East and Central Asia) are less than nonMPA in New
Zealand.1 The main contributors to these
inequalities are mortality in the older age groups (45+ years), particularly for
cancer and cardiovascular disease.1
Mortality rates change over time with changes in society,
culture and economy, and perhaps differentially by ethnicity. Māori
mortality decreased markedly up to the 1980’s then slowed its decline,
such that the gap between Māori and non-Māori increased. Since the
late 1990s, mortality rates have declined at a faster rate among Māori, and
absolute differences in rates (and life expectancy) are again closing between
Māori and non-Māori.2,3
In the past, we have only linked 3 years of mortality data
to each census to secure more rapid monitoring data after each census and also
because linkage success decayed with time following census. However, we noted in
the 2001–04 mortality linkage and in parallel linkage of 5 years of cancer
registrations to the census (CancerTrends; www.uow.otago.ac.nz/CancerTrends-info.html)
that linkage now deteriorates little with time for events 4 to 5 years after
census night.
This improvement is due to numerous sources of residential
geocodes, enabling us to better select a geocode for the decedent at about
census night even when their death occurred 4 to 5 years after census night. To
provide more complete data on social group trends in mortality in New Zealand,
we linked 2004–06 mortality records to the already existing 2001–04
census-mortality cohort.
This paper presents trends in mortality, by ethnicity, for
the period 1981–2006, with a particular focus on comparisons of the first
and second 30 months post-2001 Census (hereafter 2001–03 and
2003–06, respectively) with existing 1996–99 NZCMS data. We
investigate the validity of the extended census-cohort, and changes in
ethnic-specific all-cause and cause-specific mortality rates and inequalities.
MethodLinkage and weighting—Methods
for linkage of each of the 1981, 1986, 1991, 1996 and 2001 Censuses with 3 years
of subsequent mortality data have been described
elsewhere.4–9 A technical note on the
2004-06 linkage can be found at the NZCMS
website.10 Briefly, probabilistic record
linkage was used to link census and mortality records anonymously using the
variables sex, date of birth, country of birth and ethnic group. The blocking
variable used for matching was address or census area unit, using multiple
health dataset sources for address geocodes to attempt to bracket the date of
the 2001 Census.
Mortality records for 2001–06 eligible for
linkage were those where the decedent was alive at the 2001 Census and was
living in New Zealand on 2001 Census night according to the duration in New
Zealand variable recorded on the mortality file.
The percentage of eligible mortality records linked in
2001–04 was 79.6%, and in 2004-06 was 79.8%. Because not all mortality
records were linked to a corresponding census record, it was necessary to
correct for any linkage bias and consequent underestimation of mortality rates.
Weights were calculated based on variables that were
predictors of linkage in logistic regression analyses: age at census, sex,
prioritised ethnicity, rurality, residential mobility of area unit, Territorial
Authority, NZ deprivation index, months since census night at death, and cause
of death.
To improve efficiency of data processing, the method
for weight calculation for the full 2001–06 data differed from previous
years. Cells within a stratum that met the numerical criterion of > 5 linked
records, were separated and assigned an independent weight, whereas the
remaining cells were collapsed.
The order of collapsing of strata variables to ensure
sufficient cell sizes was based on the strength of their relationship with
linkage (see10). The initial weights were
secondarily weighted (in the same manner as previous cohorts) by age, sex and
ethnicity so that the sum of all the weighted linked mortality records was
forced to equal the total number of linked and unlinked mortality records within
age, sex and ethnicity strata. Overall, the revised weighting procedure was
found to perform as satisfactorily as previous methods
(see10 for a more detailed description of the
weighting process).
Due to larger than expected reductions in mortality
rates from 2001–03 to 2003–06 (as reported in Results section of
this paper), we undertook an additional weighting to force the exact sum of
weighted and eligible deaths for the first and second 30-month periods following
the 2001 Census to exactly agree (see 10). That
is, we were concerned that despite including ‘months since census’
in our primary weighting strategy, our above algorithm of aggregating cells did
not adequately correct for any residual deterioration in linkage success –
particularly for the younger Māori and Pacific age groups. However, there
were no systematic differences between our preferred weights and the additional
weights based on duration since census night.
We therefore concluded that residual linkage bias by
time since census was not occurring. Consequently, the preferred linkage weights
were used for all analyses by the 30 month split (i.e. 2001–03 and
2003–06). We also include previously published 2001–04 rates for
comparison.2 11
Cohort analyses—As in previous
NZCMS reports and publications (e.g.11) and
consistent with ethnicity standards,12 a
“Total” definition of ethnicity was used for Māori, Pacific and
Asian in analyses, using (all) ethnic groups self-identified by the census
question. Thus individuals could be assigned to up to three of Māori,
Pacific and Asian ethnicities. A mutually exclusive group of non-Māori
non-Pacific non-Asian (hereafter called European/Other) was used as the
reference group for ethnicity comparisons.
Age-standardised rates were calculated for all four
ethnic groups, using the WHO World population as the standard. We examined
changes in mortality rates and relative (standardised rate ratios; SRR) and
absolute (standardised rate differences; SRD) inequalities in rates by sex and
age group. All data analyses were conducted in SAS.
ResultsAll-cause mortality rates—Figure 1
shows age standardised all-cause mortality rates for 1–74 year old males
and females. Each observation is plotted at the midpoint of its period of
follow-up according to the X-axis, meaning that the 2001–03 and
2003–06 series are closer together than the 5-year gaps in the existing
1981–84 to 2001–04 series. The 2001–03 rates commencing the
new series agree reasonably closely with the previous 2001–04 rates, with
any difference being due to slightly different follow-up periods (30 versus 36
months) and the modified linkage weights.
Mortality rates were consistently highest for Māori,
followed by Pacific, then European/Other ethnicities, and lowest for Asians, and
inequalities were large at all points in time. The long-run trend of steadily
decreasing mortality rates over time for European/Other and Asian continued and
recent accelerations in Māori mortality decline seemed to be maintained.
Pacific mortality rates were more variable pre-1996, but
also exhibited the same decrease from 1996 onwards. More specifically, observed
all-cause mortality rates fell for all ethnic groups from 2001–03 to
2003–06, but more so for Pacific (18.3% and 21.7% for males and females,
respectively) and Asian (22.2%, 16.7%), than for Māori (13.2%, 14.2%) and
European/Other (13.0%, 10.4%) (Table 1 and Table 2).
Figure 1. All cause
age-standardised mortality rate (per 100,000) for males and females by
ethnicity
 The first line (starting from the left) for each ethnic
group is for the previously published 1981–84, 1986–89,
1991–94, 1996–99 and 2001–04 series (i.e. three years of
mortality data each linked to a census). The second line is that for the new
series, 2001-03 and 2003-06. Error bars are 95% confidence intervals.
Table 1. Person years and numbers of weighted deaths for all ages
(1-74 years) combined for 2001–03 and 2003–06
Table 2. Standardised mortality rates and percentage change from
preceding cohort for all cause mortality by ethnicity, period and sex. (Full
tables of rates for all age groups and all causes of death are available as web
annex tables at http://www.uow.otago.ac.nz/nzcms-info.html.)
† 2001–04 compared to 1996–99;
2001–03 compared to 1996–99; and 2003–06 compared to
2001–03,
We suspect that there is a systematic bias in the NZCMS due
to the inability to censor respondents who migrate out from New Zealand after
census night, and we also suspect this bias might be greater for Pacific and
Asian people. We consider this further in the Discussion. Thus, the
‘observed’ rates, SRRs and SRDs for Asian and Pacific people in
2003–06 need to be treated with considerable caution and accordingly, the
remainder of the Results section is mostly focussed on rates calculated for
Māori and European/Other.
Observed rate ratios for
Māori, compared to European/Other, were 2.43 (95%CI 2.31–2.57) for
males and 2.72 (2.56–2.89) for females in 2003–06, the same as in
2001–03 for males (2.44) and 7% less for females (2.84) (Table 3 and
Figure 2). The observed SRDs decreased by 13% for males (from 434 to 376 per
100,000) and by 16% for females (from 335 to 281 per 100,000).
Table 3. Age-standardised rate ratios (SRRs) and rate differences
(SRD) for selected age groups and causes of death, by sex by ethnic group, for
1996-99 onwards. (Full tables are available as web annex tables at http://www.uow.otago.ac.nz/nzcms-info.html)
Mortality rates by age
group—Mortality rates were greatest for Māori, followed by
Pacific, then European/Other ethnicities, and lowest for Asians for adults aged
15+ yrs (Figure 3; actual rates and percentage changes in Web Annex Table 1 at
www.uow.otago.ac.nz/nzcms-info.html). Across all age groups and for both
sexes, there is a long-term, steadily decreasing trend in mortality rates.
Rates for Pacific tend to show larger than plausible
decreases from 2001–03 to 2003–06 for 45–64 year olds (22.2%
and 21.8%, males and females respectively) and for 65–74 year olds (17.1%
and 18.6%). By way of comparison, the corresponding percentage declines for
European/Other were 12.4% and 7.9% for 45–64 year olds, and 13.8% and
10.4% for 65–74 year olds. Percentage declines from 2001–03 to
2003–06, were also particularly notable among these older age groups for
Asian and, to a lesser extent, Māori.
Figure 2. Trends in
relative (SRR; left) and absolute (SRD; right) inequalities in all cause
mortality for Māori, Pacific and Asian each compared to European/Other, by
sex, for all ages (1–74 years)
 The first line (starting from the left) for each ethnic
group is for the previously published 1981-84, 1986-89, 1991-94, 1996-99 and
2001-04 series (i.e. three years of mortality data each linked to a census). The
second line is that for the new series, 2001-03 and 2003-06. Error bars are 95%
confidence intervals.
Trends in relative inequalities (SRRs) between Māori
and European/Other by age group from 1996–99 to 2001–03 to
2003–06 were unclear across age groups, and therefore arguably best
assumed as stable over time (Web Annex Table 3).
Given the background trends for mortality reduction in all
ethnic by age groups, there was necessarily a pattern of reducing absolute
inequalities between Māori and European/Other over time across age groups.
For example, the SRDs among 45–64 year olds reduced by 29% for males and
21% for females from 1996–99 to 2003–06 (Web Annex Table 3),
although these may be somewhat overestimated due to the suspected migration bias
within NZCMS cohorts (see Discussion).
Mortality rates by specific cause of death—
Figure 4 shows cause-specific mortality rates. Declines in
cardiovascular disease and injury mortality were the main drivers of all-cause
mortality rate reductions for all ethnic groups (Figure 4). Relative
inequalities in CVD between Māori and European/Other remain very 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%.
 Each line plots the previously published 1981-84, 1986-89,
1991-94, and 1996-99 series (but not 2001-04), and then continues with the new
2001-03 and 2003-06 series. Error bars are 95% confidence intervals.
 Each line plots the previously published 1981-84, 1986-89,
1991-94, and 1996-99 series (but not 2001-04), and then continues with the new
2001-03 and 2003-06 series. Error bars are 95% confidence intervals.
Māori unintentional injury mortality rates are higher
than other ethnicities, for both males and females. Inequalities were unstable
over time, and not measured with sufficient precision to make confident
conclusions about trends in Māori European/Other inequalities. Injury rates
were comparable between Pacific and European/Other ethnicities since
1996–99, and lower among Asian.
Rates for other causes of death can be found at the NZCMS
website: www.uow.otago.ac.nz/nzcms-info.html
DiscussionObserved all-cause mortality rates continued to fall in
2003–06 compared to previous periods for all ethnic groups, but especially
so for Pacific and Asian people (and probably affected by bias—see below).
The falling mortality rates are largely driven by falls in CVD and unintentional
injury.
Relative inequalities in mortality between Māori and
European/Other have stabilised since the late 1990s—and possibly decreased
for females. Given this stability or slight reduction in relative inequalities,
and the overall decreasing trend in mortality for all ethnic groups, the
absolute inequalities between Māori and European/Other have necessarily
decreased.
Methodologically, we believe we have encountered a bias in
the NZCMS when results are reported by time since census. As shown in Figure 1,
Pacific and Asian rates seem to fall by an implausibly large amount from
2001–03 to 2003–06. We thoroughly checked our linkage bias weights
(see Methods and elsewhere10), and do not
believe that inadequate adjustment for (any) decline in linkage success is the
problem. Rather, we think that the number of deaths among the people alive on
census night is progressively underestimated with time since census night, due
to our inability to censor New Zealanders as they emigrate.
Put more simply, we do not identify deaths among those New
Zealand residents who completed the 2001 Census and subsequently emigrated, and
possibly died overseas. As there is relatively frequent migration to and from
the Pacific 13 14, and this likely applies to a
more recent immigration population such as Asian people, we suspect this bias is
greater among Pacific and Asian people.
A direct test of this hypothesis would require data on
emigration by ethnicity to allow censoring, or identification of deaths among
those emigrating. We do not have such data. However, we do have data on
‘permanent and long-term departures’ from New Zealand by
country/region of destination (personal communication, Robert Didham, Statistics
New Zealand, July 2010).
If our hypothesis about differential emigration by ethnicity
is correct, we might also expect emigration of older people (at great risk of
death) to be more common among Pacific and Asian populations. Those people
emigrating to the Pacific or Asia are skewed towards older age groups, both
using simple counts and also a ‘crude’ proportion measure using the
2006 New Zealand Pacific, Asian and all population count data as denominators
(Web Annex Figure 1 and 2).
If our suspicions are correct and our observed mortality
rates are a result of emigrant bias, then mortality rates for Asian and Pacific
in 2003–06 are too low, and SRRs in 2003–06 comparing either Pacific
or Asian with European/Other are also too low (due to misclassification bias of
the mortality outcome that is differential by ethnicity). We do not have enough
evidence to suggest any difference in this bias (in relative terms) for
Māori and European/Other; consequently, the SRRs comparing Māori with
European/Other are probably valid.
A possibly greater underestimation of Pacific and Asian
mortality rates is consistent with a study that examined “unhealthy return
migrant” (URM) bias mortality rates for
Pacific.15 Tobias showed that it is possible to
estimate the extent of this bias using lung cancer as a “tracer
condition” whereby spuriously high survival could only be explained by
emigration, and in turn allowed an estimate of approximately 20% undercount for
Pacific deaths in older people. However, its accuracy is unknown and
immeasurable. For example, the URM adjustor might account for both return
migration and missed NHI links between the National Cancer Registry and
mortality files.
There are potential methods to correct this bias in the
NZCMS in the future, if we are to present mortality rates by year since census
night. First, we currently only attempt to link deaths when the mortality record
states they have been in New Zealand long enough to have answered the previous
census. Thus, if we were to assume that immigration and emigration roughly
cancel each other out (not currently true for Asian people), then we could make
these excluded mortality records ‘eligible’, and when they fail to
be linked to a census record they will then contribute to a large linkage bias
weight, thus also (potentially) correcting for this emigrant bias. Second, we
could attempt to link NZCMS data to migration data, and ‘properly’
censor the cohorts. However, this would be a large undertaking, and possibly not
justifiable from either a SNZ or a funders perspective.
Third, we could attempt to create a database of New Zealand
residents (as of census night) dying overseas in the following 5 years. This
would also be a very large undertaking, and probably unreliable. Fourth, we
could use external data on emigration rates by sex, age and ethnicity to
undertake quantitative bias analyses (or sensitivity analyses) of the results
presented here.
Unfortunately, migration data is not collected by ethnic
group (the closest approximation is country of birth), and we would have to
estimate the mortality rate among the emigrants (which would not be the same as
non-emigrants). Finally, and currently our preferred option, once 5 years of
mortality data have been linked to the 2006 census, we will have a continuous
series of 10 years of linked data, with the fifth and sixth years bracketing the
beginning of the new cohort. By using regression modelling, we would be able to
estimate the magnitude of unexpected ‘jumps’ in mortality rates
across this boundary by sex, age and ethnic group, and thereby estimate sex by
age by ethnic group specific adjustment factors.
ConclusionsWe 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 to the NZCMS means mortality rates
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 Māori compared to
European/Other over this time were probably stable in relative terms and
decreasing in absolute terms, but CVD inequalities probably decreased in both
absolute and relative terms.
Statistics NZ Security Statement:
Access to the data used in this study was provided by Statistics New
Zealand under conditions designed to give effect to the security and
confidentiality provisions of the Statistics Act 1975. The results presented in
this study are the work of the author, not Statistics New Zealand’s.
Author information: Lavinia Tan, Research
Fellow; Tony A Blakely, Director; Health Inequalities Research Programme,
University of Otago, Wellington
Acknowledgements: We acknowledge comments
on drafts of this paper from Martin Tobias, Robert Didham and June Atkinson. The
New Zealand Census-Mortality Study was initially funded by the Health Research
Council of New Zealand and is now part of the HRC-funded Health Inequalities
Research Programme (08/048). The NZCMS is now principally funded by the Ministry
of Health.
Correspondence:
Tony Blakely, Health Inequalities Research Programme, University of
Otago, Wellington, PO Box 7343, Wellington, New Zealand. Fax: +64 (0)4 3895319;
email: tony.blakely@otago.ac.nz
References:
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