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Does telephone number tracing reduce non-response bias in the EU-
SILC?
Drífa Jónasdóttir, Statistics Iceland
Anton Örn Karlsson, Statistics Iceland
Bengt Oscar Lagerstrøm, Statistics Norway
Introduction
• Aim: To estimate the effect of manual phone number search on the representativity of the final sample and if the search can rectify possible bias in the survey.
• Results were compared between Norway and Iceland.
22.8.2013
Method
• EU-SILC – Longitudinal survey
• Main aim: Poverty and social exclusion in Europe • Four waves, yearly, rotating panel • Households
– Individuals sampled
• Conducted in Norway and Iceland since 2004 • First wave of 2012 used in this analysis
– Mode • CATI
– Phone numbers » Automatic search » Manual search
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Analysis
• Two main stages – Potential bias
• Distribution of answers to key questions by groups of respondents (listed/unlisted)
– Representativity of the final group of respondents • Comparision of main background variables by listed/unlisted
sample units.
• Logistic regression model to predict the likelihood of uncovering phone numbers for sample units.
• R-indicator to compare the representativity of the final group of respondents and those with registered phone numbers.
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Results: Response rates
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Response rates in Iceland and in Norway.
Norway (%) Iceland (%)
Total response rate 55,5 76,3
Response rate 1st wave 63,0 78,5
Total refusal rate 24,2 11,6
Refusal rate 1st wave 20,7 10,0
Total contact rate 86,6 91,8
Contact rate 1st wave 87,9 91,5
Results: Potential bias
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Differences, in percentages, in responses to questions on key questions.
Iceland Norway Total Listed Diff Total Listed Diff
Basic activity status
at work 66,7 66,0 0,7 61,5 61,3 -0,2
Unemployed 4,5 2,4 2,1 0,3 0,3 0,0
in retirement or early retirement or has given up business 13,7 15,3 -1,6 10,9 10,1 -0,8
other inactive person 15,2 15,3 -0,1 27,3 28,2 0,9
Actively looking for a job
Yes 21,6 19,6 2,0 6,8 6,8 0,0
No 78,4 80,4 -2,0 93,2 93,2 0,0
Capacity to face unexpected financial expenses
Yes 64,6 68,7 -4,1 90,9 91,1 -0,2
No 35,4 31,3 4,1 9,1 8,9 0,2
Dwelling type
detached house 36,1 39,2 -3,1 62,1 62,4 0,3
semi-detached or terraced house 17,6 18,7 -1,1 19,6 19,1 -0,5
apartment or flat in a building with less than 10 dwellings 13,4 12,2 1,2 4,1 4,0 -0,1
apartment or flat in a building with 10 or more dwellings 31,8 28,8 3,0 13,8 13,9 0,1
some other kind of accommodation 11,2 10,3 0,9 0,4 ,04 -0,4
Tenure status
Outright owner 20,0 22,3 -2,3 24,7 25,6 0,9
Owner paying mortgage 57,6 62,0 -4,4 60,7 60,4 -0,3
Tenant or subtenant paying rent at prevailing or market rate 12,1 7,8 4,3 7,8 7,3 -0,5
Accommodation is rented at a reduced rate (lower price that
the market price)
8,7 6,6 2,1 0,5 0,4 -0,1
Accommodation is provided free 1,5 1,2 0,3 6,4 6,1 -0,3
Results: Representativity(1)
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Differences between total and listed population variables in Iceland and Norway.
Iceland Norway
Total (%) Listed (%) Difference Total (%) Listed (%) Difference
All 100 100 100 100
Gender
Male 51,7 50,0 -1,7 51,4 51,3 -0,1
Female 48,3 50,0 1,7 48,6 48,7 0,1
Age group
16-29 22,8 20,0 -2,8 25,6 21,7 -3,9
30-44 26,4 23,8 -2,6 25,3 23,2 -2,1
45-59 26,8 29,2 2,4 23,8 25,7 1,9
60 or older 24,0 27,0 3,0 25,3 29,3 4,0
Ethnicity
Native 87,5 92,8 5,3 76,8 80,9 4,1
Foreign 12,5 7,2 -5,3 23,2 19,1 -4,1
Marital stat
Married 45,6 48,9 3,3 42,2 44,5 2,3
Not married 54,3 51,1 -3,2 57,8 55,5 -2,3
Education
Low 39,0 38,8 -0,2 29,3 28,5 -0,8
Middle 33,4 33,4 0,0 40,2 42,8 2,6
High 22,7 24,3 1,6 30,1 28,6 -1,5
Unknown 5,3 3,6 -1,7 0,3 0,1 -0,2
Results: Representativity(2)
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Logistic regression model for automatic tracing of phone numbers.
Iceland Norway
Gender (male) 0,69* 1,00
Age 1,01 1,05*
Marital status (married) 3,04* 1,12
Ethnicity (Native) 6,32* 4,47*
Education (Medium) 0,92 1,18
Education (High) 1,25* 0,77*
* < 0,05
Results: Representativity(3)
R-indicators for Iceland and Norway
Iceland Norway
Automatic 0,70 0,71
Automatic+manual 0,98 0,74
Automatic respondents 0,72 0,80
All respondents 0,88 0,88
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Results: Representativity(4)
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Partial R-indicators for Iceland and Norway.
Variable
Automatic search
partial R-indicator
Final respondents
partial R-indicator
Difference
Iceland Norway Iceland Norway Iceland Norway
Gender 0,021 0,000 0,006 0,004 0,015 -0,004
Age 0,002 0,102 0,030 0,081 -0,029 0,021
Ethnicity 0,073 0,082 0,024 0,094 0,048 -0,012
Marit. stat 0,059 0,005 0,030 0,007 0,029 -0,002
Education 0,018 0,021 0,018 0,018 0,000 0,003
Conclusions
• Manual tracing increases representativity of – final respondents – sample units with phonenumbers
• Distribution of key variables was affected by manual tracing – Especially for Iceland – Uncertain if this means we are getting closer to the
population values or not.
• Next steps: – Effects of different tracing approaches – Analysis of process efficiency.
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Thank you.
Drífa Jónasdóttir
Surveys
Resourses and Services
Statistics Iceland
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