Northern Ireland Longitudinal Study:
Linking the Northern Ireland Longitudinal Study
to the 1981 Census
NILS Working Paper 6.0
October 2015
NILS Research Support Unit
A background to the NILS is available in NILS Working Paper 1 and the NILS Matching Methodology is
available in NILS Working Paper 3.1. This paper provides an overview of the linkage of 1981 Census
data into the NILS.
1. NILS Census Data Integration
The foundation of the NILS data structure is based on NILS members selected from Health Card
Registration Data. The 1991, 2001 and 2011 Censuses are incorporated into the NILS in the
following manner.



2001 Census – Linked to NILS members considered ‘Live’ in the Health Card Data from April
2001;
2011 Census - Linked to NILS members considered ‘Live’ in the Health Card Data from April
2011;
1991 Census – Proxy 1991 Health Card dataset created from earliest available download
from April 2001:
o linked to NILS members considered ‘Live’ in the Health Card Data from April 2001;
o additional NILS members who were identified as having died between the 1991 and
2001 Censuses; and
o additional NILS members identified as having emigrated between the 1991 and 2001
Censuses.
Unlike the 1991 Census, it was identified that it was not possible to create a proxy 1981 Health Card
Registration ‘spine’ due to poor subject representation prior to 1991. It was decided to carry out
data matching based on all NILS members having a birth date prior to the date of the 1981 Census.
2. Matching Results
Using the NILS matching methodology a total of 340,066 matches between NILS members and 1981
Census data were confirmed. As the 1981 Census data integration differs to previous Health Card to
Census linkages, match rates are not directly comparable. However, comparable backward linkage
rates can be calculated. These figures are drawn from NILS members identified in the 1991 Census
link, who also have a link within the 1981 Census. Similar match rates have been drawn for other
Health Card to Census linkages. The table below shows that the 1991-1981 Census linkage is slightly
less than the 2001-1991 and 2011-2001 equivalents.
Table 1: Census Backward Linkage Comparisons
1981 (Based on 1991 Census)
1991 (Based on 2001 Census)
2001 (Based on 2011 Census)
Census Data
Count
Links
%
364,818
390,452
422,175
314,151
354,016
370,417
86.1%
90.7%
87.7%
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3. NILS Census 1981 Data Comparison – Full 1981 Census
Graphs 1-4 shows a comparison of distributions between the full 1981 Census and 1981 Census data
linked into the NILS by district, gender and age and religion.
Graph 1 shows that proportions of district information are broadly similar across all areas. Belfast
does show a slight under-representation with a difference of almost one percentage point.
Graph 1: Distribution of 1981 Census Data: District
25.0%
Total Census
NILS Linkage
20.0%
15.0%
10.0%
5.0%
Antrim
Ards
Armagh
Ballymena
Ballymoney
Banbridge
Belfast
Carrickfergus
Castlereagh
Coleraine
Cookstown
Craigavon
Down
Dungannon
Fermanagh
Larne
Limavady
Lisburn
Londonderry
Magherafelt
Moyle
Newry & Mourne
Newtownabbey
North Down
Omagh
Strabane
0.0%
Graphs 2 and 3 showing proportion distributions by gender and single year of age indicate similar
trends with under-representation evident for persons aged 60+. This can probably be explained by
the higher mortality rate in this age category.
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Graph 2 Distribution of 1981 Census Data: Male Single Year of Age
2.5%
Total Census
NILS Linkage
2.0%
1.5%
1.0%
0.5%
0.0%
0
4
8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88
Graph 3 Distribution of 1981 Census Data: Female Single Year of Age
2.5%
Full Census
NILS Linkage
2.0%
1.5%
1.0%
0.5%
0
3
6
9
12
15
18
21
24
27
30
33
36
39
42
45
48
51
54
57
60
63
66
69
72
75
78
81
84
87
90+
0.0%
Graph 4 compares proportion distributions between declared religious denominations. Each
category indicates a slight over-representation except for ‘Other Denominations’ showing a
difference of around two percentage points.
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Graph 4 Distribution of 1981 Census Data: Religious Denomination
Total Census
NILS Linkage
30.0%
25.0%
20.0%
15.0%
10.0%
5.0%
0.0%
NISRA
October 2015
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