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| Journal of the New Zealand Medical Association,
06-June-2003, Vol 116 No 1175
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|
Unlocking the numerator-denominator bias III: adjustment
ratios by ethnicity for 1981–1999 mortality data. The New Zealand
Census-Mortality Study
Shilpi Ajwani, Tony Blakely, Bridget Robson, June Atkinson
and Cindy Kiro
A focus on reducing health inequalities between ethnic
groups is both a legislative and policy imperative for the health sector of
Aotearoa/New Zealand.1 The Government has
committed itself to reducing health inequalities, especially those experienced
by Maori and Pacific peoples.2 In an
evidence-based health policy environment, data quality underpins the ability of
indigenous peoples to obtain collective
wellbeing.3 The Crown has a Treaty
responsibility (of good governance) to maintain Maori health data collections to
at least the same quality as those of non-Maori.
Over the past quarter century, the definition of ethnicity
used in the New Zealand Census has changed from a biological concept to one of
cultural affiliation.4 However, this was not
the case with mortality data, which rely on the death registration form. It is
only since September 1995 that the ethnicity question on the death registration
form has been consistent with that on the (1996) census form. Furthermore, there
is a difference between the data collection methods used in the death
registration and the census forms (eg, funeral-director-elicited versus
self-identified). These discrepancies have created major problems for monitoring
ethnic mortality trends over time.
Anonymous and probabilistic record linkage of census and
mortality data in the New Zealand Census-Mortality Study (NZCMS) allows a direct
comparison of the self-identified ethnicity at the previous census and the
ethnicity recorded in the mortality data. Consequently, it is possible to
quantify or ‘unlock’ the numerator-denominator bias that causes
ethnic-specific mortality rates to be calculated incorrectly. We have previously
determined that under-counting of Maori and Pacific deaths in 1991–1994
mortality data (numerator) relative to the 1991 Census (denominator) causes a
severe underestimate of Maori and Pacific mortality
rates.5
The objective of the current study is to additionally
estimate the numerator-denominator bias for 1981–1984, 1986–1989,
and 1996–1999, allowing a determination of trends in bias for the 1980s
and 1990s. The findings of this study subsume those of the earlier study of bias
in 1991–1994 census-mortality records5
and present analyses for all four (1981–1984, 1986–1989,
1991–1994, and 1996–1999) census-mortality cohorts. This study uses
‘age at death’ (instead of ‘age at census’ as used in
the previous study) and an improved method for weighting.
MethodsThe methodology has been
described in detail in technical
reports.6–8 Briefly, approximately three
quarters of eligible mortality records (aged 0–74 years at census night)
for three years subsequent to each of the 1981, 1986, 1991 and 1996 Censuses
were anonymously and probabilistically linked to census records. For the
purposes of determining numerator-denominator bias, the linked records were
further restricted to highly probably links (HPL), where ethnicity had no effect
on linkage probability. In the previously reported 1991–1994
numerator-denominator analyses, the HPL were restricted to records with
agreement on meshblock (ie, small areas of residence containing approximately
100 people).5 In this study, we also included
links with agreement on area unit (approximately 2000 people) in our HPL data
sets to increase linkages for rural decedents that seldom had a meshblock
assigned. The number of mortality records in the HPL data set (and as a
percentage of eligible mortality records) for 1981–1984, 1986–1989,
1991–1994, and 1996–1999, respectively, were 28 470 (64%), 30 891
(69%), 30 789 (75%), and 29 637
(75%).
Ethnicity definitions Until September 1995, the death register classified each decedent as one of three groups: NZ Maori (½ or more Maori ‘blood’); Pacific Island (more than ½ Pacific Island ‘blood’); or Other (non-Maori non-Pacific). After September 1995, the death register collected data on ‘ethnic group’ and coded up to three ethnic groups per person. An alternative (although not commonly used) source of ethnicity information for deaths is the National Health Index (NHI), from which records from 1988 onwards can be linked to the mortality data by way of the unique NHI number. The censuses collected data on ‘ethnic origin’ in 1981 and 1986 and ‘ethnic group’ in 1991 and 1996. Each census (and post-1995 mortality data) allowed for multiple groups to be self-identified, thereby allowing an assignation of ‘sole’ and ‘prioritised’ ethnic groups:
The ethnicity question on the
1996 Census was modified to include extra categories and encouraged multi-ethnic
responses. Consequently, the 1996 Census ‘sole’ groups became
smaller and the ‘prioritised’ groups became larger compared with the
1991 Census. We also categorised an Asian group for 1996–1999 data, with
the ‘prioritised Asian’ group being assigned if one of the
self-identified ethnic groups was Asian
and they had no Maori or Pacific
self-identified ethnic groups.
Determining the numerator-denominator bias A major reason for determining the numerator-denominator bias is to correct mortality rates by ethnicity. From 1981 to 1985, official Maori and Pacific mortality rates were calculated using the ½ or more Maori (or Pacific) ‘blood’ categorisation in both the deaths and census data. Between 1986 and 1995, sole ethnic group from the census was used as the denominator, with the ½ or more Maori (or Pacific) ‘blood’ category in the mortality data. From 1996 onwards the Ministry of Health has used prioritised ethnicity for both census and mortality data to calculate rates. Therefore, we present numerator-denominator bias results in this paper for:
For
completeness, we also present adjustment ratios comparing the sole populations
in 1996–1999.
The numerator-denominator bias was determined by cross-classification of the weighted HPL data set by census and death registration ethnicity. (The weights make the HPL data set representative of all eligible mortality records; their calculation is described in detail elsewhere.7) This cross-classification was also conducted by strata of age at death, regional health association (RHA), and rurality, in order to determine heterogeneity of the numerator-denominator bias. The bias, calculated as an ‘adjustment ratio’ that could be applied to historic mortality data, equals the estimated number of census respondents of the given ethnic group divided by the number of mortality records of the given ethnic group. It is possible that our calculations may be biased due to the HPL data set excluding 25% to 30% of eligible mortality records. Extensive sensitivity analyses for Maori presented elsewhere suggest that any such bias was minimal.6,7 Briefly, the key assumption for the validity of the adjustment ratio calculations is that within each stratum of mortality record ([ethnicity] by [sex] by [age at death] by [RHA] by [NZDep] by [rurality]) the distribution of census self-identified ethnicity was the same for mortality records included in the HPL data set as it was for all eligible mortality records in the given stratum. As most Maori identified as such according to mortality data also self-identified as Maori on census data, it is only among non-Maori according to mortality data that any residual bias may arise. We tested this possibility for 1991–1994 by re-calculating unlock ratios under the (somewhat extreme) scenario that non-Maori non-Pacific decedents excluded from the HPL data set (meshblock-only links) were 15% more likely to self-identify as sole Maori on the census than those included in the final data set within all strata used for weighting. Our (previous5) overall estimate of the Maori adjustment ratio of 1.29 became 1.33, or 1.31 if a 7.5% rather than a 15% estimate was used.6 The analyses in this current paper included census-mortality links formed at the census-area-unit level as well as the meshblock level, resulting in an overall Maori unlock ratio of 1.32. Thus, based on the more inclusive HPL data set used in this current paper, and further sensitivity analyses published elsewhere,6,7 we conclude that the adjustment ratios presented in this paper are reasonably accurate. ResultsThe overall adjustment ratios for
the 1981–1984, 1986–1989, 1991–1994 and 1996–1999
census-mortality cohorts are shown in Table 1. In the 1981–1984 cohort,
for example, 3615 of a total of 44 703 deaths were estimated as having
identified as sole Maori on the 1981 Census; 16% more than the 3108 recorded as
Maori on the death registration form. Therefore, the Maori mortality data for
1981–1984 would have to be multiplied by the adjustment ratio of 1.16 (ie,
3615/3108) to make them comparable with the sole Maori population from the 1981
Census.
Table 1. Census ethnicity
and death-registration-form ethnicity totals for 0–74 year old decedents
during 1981–1984, 1986–1989, 1991–1994 and
1996–1999
All the numbers are weighted, and then random rounded
to a multiple of three as per Statistics New Zealand protocol. Minimum cell size
is 6. *The census to mortality ratio is the census total divided by the death
registration form total eg, for Maori sole ethnicity 1.32 is the correction
factor to apply to 1986–1989 ethnic specific mortality rates calculated
using sole ethnicity as the denominator.
The Maori adjustment ratio in both 1986–1989 and
1991–1994 was 1.32 using the census ‘sole’ categorisation.
With the change in ethnicity data collection on death registrations in 1995, the
ratio for sole Maori deaths reversed to 0.86 for the 1996–1999 cohort.
However, using the ‘prioritised’ classification in 1996–1999,
the study found 7% more decedents identified as Maori on census than on death
register (a ratio of 1.07). (Currently published Maori mortality rates for the
late 1990s use the ‘prioritised’
classification,9 and are therefore still
modestly underestimating Maori mortality rates.) For Pacific peoples, the trend
over time was similar for the first three cohorts, although the bias was greater
than for Maori, with ratios of 1.55, 1.76, and 1.68 respectively. Like Maori,
Pacific deaths were over-reported according to sole ethnicity in the
1996–1999 cohort, but little bias was observed for the prioritised Pacific
ethnic group with a ratio of 0.99.
*ratios for Asian are calculated in the 1996–1999
cohort only; †ratio is for non-Maori
non-Pacific for the first three cohorts, and is for non-Maori non-Pacific
non-Asian in the 1996–1999 cohort only
There was little variation in the adjustment ratios by sex,
except for Pacific people in the 1981–1984 cohort (1.48 for men vs 1.68
for women). The Pacific ratios by sex were, however, unstable due to the smaller
number of linked records and must be treated with caution.
By age, the Maori adjustment ratios were substantially
greater at younger age groups, at least in the 1981–1984, 1986–1989,
and 1991–1994 cohorts (Table 2). For example, the underestimation of
deaths among the 0–14 year age group in the respective three cohorts was
34% ([0.52/1.52] x 100), 36% and 44%, compared with 11%, 15% and 19% among the
65–77 year age group. There was no clear pattern by age for Maori in the
1996–1999 cohort, but young adults (prioritised ethnicity) had the highest
under-reporting with a ratio of 1.13. No pattern by age was observed for Pacific
people. The Pacific ratios were higher for all age categories in the first three
cohorts and close to one in the final cohort.
Table 3. Census ethnicity to
death-registration-form ethnicity ratios by Regional Health Authority
(RHA)
*ratios
for Asian are calculated in the 1996–1999 cohort only;
†ratio is for non-Maori non-Pacific for
the first three cohorts, and is for non-Maori non-Pacific non-Asian in the
1996–1999 cohort only
There was variation in the numerator-denominator bias for
Maori by region (Table 3), being greatest for the Central and Southern RHA with
adjustment ratios of 1.34 for the 1981–1984, 1.90 for the 1991–1994,
and 1.13 for the 1996–1999 cohort. However, an aberration was noted for
the 1986–1989 cohort, when the greatest bias for Maori was in the Northern
RHA. Further investigation of the northern region for 1986–1989 disclosed
a major, localised problem. At the Onehunga Registration Office, 908 deaths were
reported during 1981–1984, of which 280 were Maori (approximately 30%). Of
the total 1408 deaths reported in that office in 1986–1989, therefore,
approximately 420 should have been recorded as Maori, but only 18 Maori deaths
were reported.
There were very few Pacific (or Asian) deaths reported in
rural areas, and the urban adjustment ratios for Pacific and Asian deaths were
very similar to the overall ratios reported in Table 1. Hence, we report urban
and rural ratios for Maori and non-Maori only. As observed in Table 4, the
underestimation of Maori deaths was higher in urban areas than in rural, and the
pattern was similar across all four cohorts.
The adjustment ratios for Asian deaths were available only
for the 1996–1999 period, and were close to one without any substantial
variation by other demographic factors.
A comparison using ethnicity from the National Health Index
(NHI) file, instead of the death-registration-form ethnicity, found that the NHI
ethnicity data were more accurate than the mortality data for the
1991–1994 cohort. For the 1996–1999 cohort, the NHI ethnicity data
were reasonably accurate for the sole-Maori group (with a ratio of 1.05), and
somewhat less accurate for the sole-Pacific group (ratio of 1.14). However, for
the prioritised-Maori and -Pacific ethnic groups, the NHI data were considerably
less accurate than the mortality data for 1996–1999. The census to NHI
ratios for 1996–1999 using ‘prioritised’ categorisations were
1.38 for the Maori ethnic group and 1.30 for Pacific peoples (compared with 1.07
and 0.99 respectively for the death registrations).
More detailed results, including the results for
numerator-denominator bias at two levels of stratification and ratios using
other ethnicity combinations (census sole by mortality prioritised, and census
prioritised by mortality sole for the 1996–1999 cohort, and NHI file
ethnicity) are published in a technical
report.7
DiscussionMaori and Pacific deaths were
substantially underestimated during the late 1980s and early 1990s.
Approximately one third more decedents self-identified as sole Maori on census
data than were identified as Maori on mortality data for this period, and
approximately two thirds more decedents self-identified as sole Pacific. The
change to compulsory recording of ethnicity, through incorporation of the (1996)
census question on mortality data following September 1995, caused a dramatic
reduction in numerator-denominator bias. During the late 1990s, only 7% more
decedents self-identified as prioritised Maori on census data than were
identified as prioritised Maori in mortality data. There was negligible
remaining bias for prioritised Pacific deaths. However, sole Maori and sole
Pacific counts on mortality data are overestimated in the late 1990s relative to
census data.
We believe our quantification of numerator-denominator bias
is reasonably accurate. First, our results are based on attempted record linkage
for all mortality records in the three-year period after each census. Second,
whilst only about three quarters of mortality records were linked to a census
record, we were able to use weighted analyses of these linked records to
estimate the numerator-denominator bias for all eligible mortality records.
Extensive sensitivity analyses about our methods and results are published
elsewhere.6,7
It is clear from our findings that there was large
numerator-denominator bias, especially for Pacific people, younger Maori and
Maori living in the Central and Southern RHA at least prior to 1996, even when
using sole ethnicity populations as a denominator. As a result, both Maori and
Pacific mortality rates and ethnic mortality disparities have been severely
underestimated for these periods. For example, an unadjusted relative risk of
1.0 for Maori children aged 0–14 years compared with non-Maori non-Pacific
in the early 1980s corresponds to an adjusted relative risk of 1.0 x 1.52/0.89 =
1.71 (where 1.52 is the adjustment ratio for Maori 0–14 year olds and 0.89
is the adjustment ratio for non-Maori non-Pacific 0–14 years olds (Table
2)). This large underestimation of ethnic inequalities among the young during
the 1980s and 1990s needs further consideration. Social and economic policies
implemented during that period have resulted in increasing inequalities between
Maori and non-Maori in education, income and employment. The impact of such
policies on the health of Maori and Pacific children and young people may not
have been given due consideration given that the extent of those inequalities
was not accurately represented in official statistics. The current trend of
increasing ethnic disparities in young people means that improving the living
circumstances of Maori and Pacific families is an urgent priority.
The failure to accurately record deaths has policy and
research implications beyond the practices of data collection and processing.
Maori researchers have argued that such under-counting is tantamount to a form
of institutional racism (even if unintentional by individual agents),
effectively rendering the problem of ethnic disparities
invisible.10
More accurate reporting of Maori deaths in rural areas than
in urban areas was observed in this study. Previous reports that life expectancy
was higher for urban Maori than for rural Maori, while the opposite trend was
reported for non-Maori, should be reconsidered in light of this new
finding.9
A north-south variation in bias was observed for Maori
mortality – a finding consistent with previous
studies.11,12 This regional variation in
ethnicity data quality should be particularly noted by those involved in
researching, monitoring or reporting time trends in ethnic differences by region
or in developing policies based on regionally specific data (eg, DHB funding
formulae, calculation of population estimates, priority setting, health-service
planning and monitoring of inequalities). The possibility of a north-south
gradient in ethnic misclassification in other data collections, such as hospital
admissions, should also be considered. The reasons why such a trend exists need
investigation, but potential factors might include:
Beyond these potential reasons why such a
regional trend exists, the matter of whether or not these variations could be
indicative of other regional issues that impact on the health of Maori (and, if
found to be true, in what way) requires investigation.
Our findings show that it is possible to rapidly improve
ethnicity data quality through the implementation of a consistent and compulsory
ethnicity question (ie, the change in September 1995 to include the census
ethnicity question on mortality data). Ethnicity data collections from
hospitals, primary care, cancer registrations, public health data and other
registers, such as cervical screening and immunisation, may also benefit from
ensuring the ethnicity question is standardized and compulsory for all
administrative interactions. Given that the 2001 Census ethnicity question will
remain in the 2006 Census and will therefore form the basis of denominator data
until 2010, health services should now invest in implementing the standard
question in their data collection systems.
Conversely, the finding that sole Maori are now
overestimated in mortality data indicates that there are still improvements to
be made in death registrations in the identification and recording of multiple
ethnicity. Some funeral directors report that their data processing systems
allow only a single ethnic group to be
recorded.14 This would result in
under-recording of those with multiple ethnic groups, and prevents accurate
monitoring of mortality trends for the sole-Maori population, the group that
currently experiences the highest mortality disparities. Furthermore, the
mortality rate for the Maori ethnic group (prioritised) was still underestimated
by 7% on a national level during the late 1990s, with a higher under-count in
the central/southern region (13%) and in young adults (13% for 15–24
years, and 10% for 25–44 years). This may seem minor when compared with
the severity of the previous problem but needs to be addressed
nevertheless.
The current legislative and policy imperatives to reduce
inequalities by improving the health of Maori and Pacific
peoples1,2,15,16 necessitate accurate recording
of ethnicity data across the entire health sector. This allows benchmarks to be
set and the impact of policy changes to be monitored, along with the
contributions of various parts of the sector to reducing inequalities. Given the
current commitment by the Crown to reducing inequalities in health it is
important that improvements to ethnicity data quality be made quickly and
comprehensively. This would enable the monitoring of actual outcomes rather than
artefactual effects of changes in data quality.
Principles of good practice in ethnicity data collection
need further promotion among funeral directors as well as throughout the health
sector and the general public. These include information about what is
‘ethnicity’, that it is self-identified (not based on
‘blood’ quantum), that it is possible for a person to identify with
more than one ethnic group, and that ethnic identification may change throughout
one’s life. Families could also be alerted to their right to fill in the
death registration form themselves. Areas that require extra attention include
training of data collectors, informing the public about ethnicity and their
rights to accurate information being recorded, data input and output systems,
and ongoing audit to ensure standards of collection are maintained. Our study
found that ethnicity recording on mortality data is now more accurate than that
on the National Health Index. Hospital data collections need improvement and
primary care organisations should be cautious of obtaining ethnicity data for
their patients from the NHI, which still under-reports Maori (and Pacific)
patients.
While this research does not address specific policy
interventions, the underestimation of Maori and Pacific mortality identified by
this research provides increased evidence to support strong policy imperatives
for addressing health inequalities and reinforces the need for proactive
strategies to address premature mortality among Maori and Pacific peoples.
Through such actions, the Treaty risk from ‘inaction in the face of
need’ will be minimised and Treaty rights to collective wellbeing
maximised.
Using the quantification of numerator-denominator bias
presented in this paper, we are currently re-calculating mortality rates and
trends by ethnicity over the last twenty years. Only then can we accurately
determine if ethnic mortality ‘gaps’ are closing or
widening.
Author information:
Shilpi Ajwani, Research Fellow; Tony Blakely, Senior Research Fellow;
Bridget Robson, Research Fellow (Te Ropu Rangahau Hauora a Eru Pomare); June
Atkinson, Data Manager/Biostatistician, Department of Public Health, Wellington
School of Medicine and Health Sciences, University of Otago; Cindy Kiro, Waiora:
Centre for Public Health Research, Massey University, Auckland
Acknowledgements:
The New Zealand Census Mortality Study (NZCMS) is conducted in collaboration
with Statistics New Zealand and within the confines of the Statistics Act 1975.
The NZCMS is funded by the Health Research Council of New Zealand, with
cofunding from the Ministry of Health.
Summary Statistics New
Zealand security statement: The New Zealand Census Mortality Study
(NZCMS) is a study of the relationship between socioeconomic factors and
mortality in New Zealand, based on the integration of anonymised population
census data from Statistics New Zealand and mortality data from the New Zealand
Health Information Service. The project was approved by Statistics New Zealand
as a Data Laboratory project under the Microdata Access Protocols in 1997. The
data sets created by the integration process are covered by the Statistics Act
and can be used for statistical purposes only. Only approved researchers who
have signed Statistics New Zealand’s declaration of secrecy can access the
integrated data in the Data Laboratory. A full security statement is published
in a technical report at the NZCMS web site http://www.wnmeds.ac.nz/nzcms-info.htm.
For further information about confidentiality matters in regard to this study
please contact Statistics New Zealand.
Correspondence: Dr
Tony Blakely, Department of Public Health, Wellington School of Medicine and
Health Sciences, University of Otago, P O Box 7343, Wellington. Fax: (04) 389
5319; email: tblakely@wnmeds.ac.nz
References:
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