The northern ireland longitudinal study data linkage research potential and application
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The Northern Ireland Longitudinal Study: data linkage, research potential and application. Gemma Catney Centre for Public Health, Queen’s University Belfast Meeting of the Royal Statistical Society Leeds/Bradford, 26 th January. Presentation outline.

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The Northern Ireland Longitudinal Study: data linkage, research potential and application

Gemma Catney

Centre for Public Health,

Queen’s University Belfast

Meeting of the Royal Statistical Society Leeds/Bradford, 26th January

Presentation outline

  • Part one: The NILS – background to the data and their linkages

  • Part two: Research application – Segregation and health in NI

    • Segregation and (ill)health

      • Minority/majority status

    • ‘Religious’ concentration in Northern Ireland

    • Hypotheses, data, methods

    • Results

      • Logistic regression (outcome: poor GH), pop. concentrations

      • Cox proportional hazards (outcome: all-cause mortality)

      • Segregation indices and poor general health

    • Discussion, conclusions, future work

Part one: The NILS – background to the data and their linkages

Background to the NILS and NIMS

  • Research-Driven

    • Cross-sectional studies: no information on change over time

    • Other UK LS

    • Other international mortality-based LS

    • Health and socio-demographic profile of NI

  • Legislation

    • Confidentiality protected, and managed by NISRA, under Census legislation

  • Funding

    • Infrastructure funded by the Health and Social Care R&D Division and NISRA

    • Research support function funded by ESRC and NI Government (OFMDFM)

Overview of the NILS and NIMS

  • Northern Ireland Longitudinal Study (NILS) – c. 28% representative sample of NI population (c. 500,000), based on health card registrations, linked to:

    • 2001 Census returns

    • vital events (births, deaths and marriages)

    • demographic & migration events

    • distinct Health & Care datasets

  • Northern Ireland Mortality Study (NIMS) - enumerated population at Census Day (c.1.6 million), linked to:

    • 2001 Census returns

    • subsequently registered mortality data

      Both NILS and NIMS linked to contextual and area-based data:

    • capital value of houses and property attributes

    • geographical indicators

    • settlement classifications

    • deprivation measures

Datasets routinely linked

Research based on the NILS/NIMS

  • Health and mortality

    • Temperature-related mortality and housing

    • Socio-demographic and area correlates of suicides

    • Distribution of cancer deaths in Northern Ireland by population and household type

    • Variations in alcohol-related deaths in Northern Ireland

  • Demographic trends

    • Fertility in the short-run in Northern Ireland

    • Lone mothers at time of birth: who are they?

    • Fertility levels and future school populations

    • Describing and modelling internal migration

    • Deprivation & ill-health: a comparison of Scotland and NI

  • Education, employment and income

    • Unemployment and permanent sickness in NI

    • Pervasive area poverty: modelled household income

    • House value as an indicator of cumulative wealth in older people

  • Area-based analyses

    • Rural aspects of health

    • Population movement and the spatial distribution of socio-economic and health status

    • Residential concentration/segregation and poor health

NILS Research Support Unit

Based at the Centre for Public Health (QUB) and NISRA HQ (McAuley House)

Support: 2 full-time and 1 half-time Research Support Officers

Established April 2009


raise awareness of the NILS research potential;

assist with development of research ideas and projects;

facilitate access to NILS data;

training & advice in use and analysis of NILS datasets;

promote policy relevance; and

enhance NILS research capacity

Research support

Matching process

NIMS database based on 1.6m pop. at 2001 Census

GRONI deaths data added to NIMS database on a six-monthly basis

3-stage matching process:

exact computer matching

fuzzy computer matching

detailed manual searching

Linkage rates close to 100% not possible for NIMS – why?

Non-enumeration at Census:

One Number Census methodology: imputation for adjusted est. total

Imputation varies by age, gender and geographical area

In NI enumerated 2001 Census total was 1,603,641 - an additional 81,626 people were imputed = overall imputation rate of 4.6%.

People who came to NI after 2001 and subsequently died: selective unrecorded migration

Differences between the info collected on census form and death certificate

Record Linkage: Issues and Biases

Study on potential biases:

O’ Reilly, D., Rosato, M. & Connolly, S. (2008) Unlinked vital events in census-based longitudinal studies can bias subsequent analysis.

Journal of Clin. Epid. 61: 380-385.

What are the characteristics of people whose events are not linked into the LS datasets?

What does this mean for analyses using the LS?

Record Linkage: Issues and Biases

Record Linkage Rates 2001-2005

  • 59,396 deaths available to be linked from 2001-2005

  • 6% deaths (3,392) could not be matched

Characteristics of matched and non-matched deaths

Based on data from death records (multivariate log reg):

Year of registration

Socio-demographic details

age, sex, marital status, social class (NS-SEC)

Place of death

home, hospital, nursing/residential home

Area in which they lived (SOA)

Deprivation (income domain)


Population density


Cause of death

Variation according to demographic characteristics (Outcome: unmatched death, 2001-2006)

*** P<0.001; ** P< 0.01; * P<0.05

Variation according to relative deprivation (Outcome: unmatched death, 2001-2006)

*** P<0.001; ** P< 0.01; * P<0.05

Variation by cause of death (Outcome: unmatched death, 2001-2006)

Research conclusions: small proportion of events are not linked – biases:

increase in months immediately after Census Day 2001

increase with ‘distance’ from the Census

are non-random and more frequent in …

younger males, older females

people who are perhaps more socially isolated

amongst residents of nursing/residential homes

deprived areas

where enumeration is low

Non-linkage may limit the ability to study some causes of death and potentially lead to an underestimationof social gradients

Record linkage: issues and biases

Statutory obligation to record death events. Complete & good quality data:

long experience of use for mortality analyses andthere will be biases in every linkage study ≠100%: this research shows that biases can be quantified

Small number problems i.e., falling death rates, population sub-groups (minority ethnics), cause-specific mortality (suicides, trauma & specific cancers)

yet: can increase length of follow-up study, aggregate sub-populations & increase cohort size

So there are potential biases, however...

Part two: Research application – segregation and health in NI


The help provided by the staff of the Northern Ireland Longitudinal Study and Northern Ireland Mortality Study (NILS and NIMS) and the NILS Research Support Unit is acknowledged. The NILS and NIMS are funded by the Health and Social Care Research and Development Division of the Public Health Agency (HSC R&D Division) and NISRA. The NILS-RSU is funded by the ESRC and the Northern Ireland Government. The authors alone are responsible for the interpretation of the data.

Corresponding author: [email protected]

More information on NILS/NIMS data:

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