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Background Notes

Background Notes

CSO statistical publication, , 11am

Background Notes

Purpose of Survey

The primary focus of the Survey on Income and Living Conditions (SILC) is the collection of information on the income and living conditions of different types of households in Ireland, in order to derive indicators on poverty, deprivation and social exclusion.  It is a voluntary survey (for selected households).  Up until 2021 the SILC was carried out under EU legislation (Council Regulation No 1177/2003) and commenced in Ireland in June 2003. On 01/01/2021 Council Regulation No 1177/2003 was repealed by Regulation (EU) 2019/1700.

Impact of the COVID-19 pandemic

The impact of the COVID-19 pandemic in terms of the operation of the SILC survey and on the subject matter itself is wide-ranging and is difficult to fully evaluate. Some key aspects that may have influenced the comparative results from 2020 and 2021 in this report are as follows:

Operational changes: Prior to the COVID-19 pandemic, respondents were recruited and all interviews were conducted in person. Interviewers would visit houses, often a number of times, to encourage chosen respondents to take part. In March 2020 as the restrictions began, survey interviewing was paused for a couple of weeks and then moved to telephone interviewing (CATI) for the safety of both respondents and interviewers. The survey was quickly adapted for CATI and interviewers were set up to work from home. While we had contact details for waves 2 to 5, wave 1 respondents were recruited by post, inviting them to contact us. For 2022, data collection was a mix of CAPI and CATI. These types of mode adjustments can have a significant impact on the distribution of the achieved sample. Indeed, we did see a distributional change in the tenure status for the responding households, for which we made an adjustment in the weighting. While the weighting process overall is designed to reflect society as a whole as much as possible, caution should be applied when making comparisons over time.

Behavioural changes: While temporary, societal adjustments in response to the pandemic and the consequential restrictions during 2020 and 2021 impacted the results of the survey in many ways. Some questions in SILC are subjective in nature and appear to have been influenced by the temporary conditions respondents found themselves in during lockdown. For example, the proportion that said they were Unable to afford a morning, afternoon or evening out in the last fortnight decreased 3.6 percentage points, from 7.4% in 2020 to 3.8% in 2021, while the proportion that were Unable to afford to have family or friends for a drink or meal once a month decreased from 10.5% in 2020 to 8.6% in 2021.  Though these indicators are designed to focus on affordability, the socially-oriented nature of them may have been influenced by restrictions. Another example could be Principal Economic Status (PES), where a respondent may or may not have considered themselves Unemployed while on an income support scheme. The nature of many of these changes appear to have been temporary and should be interpreted with caution.

Break in Time Series

SILC has been running as an annual survey in Ireland since 2003. The changes in regulation across household surveys introduced by Regulation 2019/1700 provided us with an opportunity to review and revise SILC methodology throughout the collection, processing and analysis phases of SILC production. We introduced these changes for the 2020 SILC survey, and therefore the year 2020 represents a break in series for the survey. See Information Note - Break in Time Series SILC 2020.

Reference Period

Information is collected from January to June with household interviews being conducted on a weekly basis. The income reference period for SILC is the previous calendar year. Therefore, the income referenced for the 2022 survey spans the period from January to December 2021.  In 2022, the achieved sample size was 4,660 households and 11,393 individuals.

Timeliness

The deprivation data for SILC 2022 was published in November 2022.

Methodology

Rotational Sample Design

In 2022 the SILC sample moved from a 5-year to a 6-year rotational sample, with both a cross-sectional and a longitudinal element. Households interviewed for the first time are Wave 1 households.  Households who are interviewed in subsequent years are Wave 2 households (2nd year in the sample), Wave 3 households (3rd year in the sample), Wave 4 (4th year in the sample), Wave 5 (5th year in the sample), or Wave 6 (6th and final year in the sample). The initial sample design attempts to seed the sample with 20% for each new wave. However, due to non-response and sample attrition the waves are not evenly balanced in the sample with Wave 1 households usually tending to dominate. 

Response Rates

The overall response rate for the SILC survey in 2022 was 39.4%.  The response rate is heavily influenced by the Wave 1 response rate which was 23.0% in 2022.  The response rates tend to be a lot higher for Wave 2-6 households and in 2022 the response rate for Wave 2-6 households was 63.4%.

Sample Design

In 2022 a new sampling methodology was introduced to ensure SILC will be able to meet the precision requirements specified in the IESS regulation. Wave 1 was selected using this methodology in SILC 2022. Waves 2, 3, 4 and 5 comes from the 2018 sampling frame, while wave 6 comes from the 2014 sampling frame, using the previous sample selection methodology in place since 2014.

The following is a brief overview of the revised SILC sample methodology, from which wave 1 of SILC 2022 was selected:

  • The SILC sample is a Stratified Simple Random Sample (SSRS).
  • The sample is stratified by county and 10 equivalised income bands.
  • Households are selected using Neyman allocation.
  • The sampling frame is the 2016 Census, excluding households previously sampled for other social surveys.
  • Including longitudinal cases from the older sample selection methodology (waves 2-6), a target of 12,000 households are selected for interview.

The following is a brief overview of the 2014 SILC sample methodology, from which waves 2-6 of SILC 2022 were selected:

  • The SILC sample is a multi-stage cluster sample resulting in all households in Ireland having an equal probability of selection.
  • The sample is stratified by NUTS4 and quintiles derived from the Pobal HP (Haase and Pratschke) Deprivation Index.
  • In the 2018 sample the clusters are based on Census Enumeration Areas, rather than the Household Survey Collection Unit Small Areas used in the 2014 sample.
  • A sample of 1,200 blocks (i.e. Census Enumeration Areas, Census 2016) from the total population of blocks is selected.
  • Blocks are selected using probability proportional to size (PPS), where the size of the block is determined by the number of occupied households on Census night 2016. 100 households from each block are selected at random to be retained for selection within each block.
  • All occupied households on Census night 2016 within each block are eligible for selection in the SILC sample.
  • Households within blocks are selected using simple random sampling without replacement (SRS) for inclusion in the survey sample.

Weighting

A design weight is assigned to each household which is calculated as the inverse proportion to the probability with which the household was sampled. For SILC, the probability of the selection of a household is based on two elements: the probability of the selection of a block, and the probability of selection of a household within that block. The design weights were calculated for Wave 1 households each year as outlined above.

Design weights are adjusted each year for each wave separately for non-response to bring the weights up to the current year. These weights are combined and scaled back and then calibrated to population totals for the current year. 

In accordance with Eurostat recommendation, CALMAR was used to calculate the household cross-sectional weights. Benchmark information was used to gross up the data to population estimates. The benchmark estimates were based on:

  • Age by sex: Individual population estimates are generated from population projections from census data. Age is broken down by 5-year age groups into seventeen categories.
  • Region: Household population estimates in each of the eight NUTS3 regions are generated using Labour Force Survey (LFS) data.
  • Household composition: Household composition estimates are also generated from the LFS. The following categories are used:
    • 1 adult aged 65+, no children under 18
    • 1 adult aged <65, no children under 18
    • 2 adults, at least 1 aged 65+, no children under 18
    • 2 adults, both aged <65, no children under 18
    • 3+ adults, no children under 18
    • 1 adult, 1+ children under 18
    • 2 adults, 1-3 children under 18
    • Other households with children under 18
  • Tenure status: Due to the COVID-19 pandemic the data collection process for SILC changed (see section above on the impact of the COVID-19 pandemic), the tenure distribution of the resulting achieved sample differed from previous SILC surveys. In the absence of a robust external benchmark, we applied 2020 tenure distributions to the 2022 weighting process using the following categories:
    • Owner-occupied: without outstanding mortgage
    • Owner-occupied: with outstanding mortgage
    • Rented, in receipt of HAP/RS/other rent subsidy
    • Rented, Local Authority
    • Rent free
    • Rented, without state housing assistance

Due to the “integrative” calibration method, the personal weight generated in CALMAR is equal to the household weight. Because there is no individual non-response within a household, the weights for personal cross-sectional respondents aged 16 and over are the same as the overall personal weight.

Precision estimates and statistical significance

Estimates were calculated in SAS using the Jackknife and the Taylor Linearisation methodology.  For the mean equivalised net disposable income, the ‘At Risk of Poverty’ rate, the ‘Deprivation’ rate and the ‘Consistent Poverty’ rate, the Jackknife Method in PROC SURVEYMEANS was used. The Taylor Linearisation Method in PROC SURVEYMEANS was used to measure the precision of the quantiles. 

SAS routines and macros were developed to calculate the precision of the more complex statistics, i.e. the Gini Coefficient and the Quintile Share Ratio (QSR), using the Jackknife Method.  The variance of the Gini and the QSR was estimated using the methodology outlined in Lohr1 Ch. 9 (Variance Estimation in Complex Surveys).  The calculations of the precision estimates took into account the weighting, the complex structure of the sample, (i.e. the fact that the sample was a cluster sample as opposed to a simple random sample) and other complications arising from the methods adopted.

When measuring the year-on-year change of a statistic, we take into account both the variance of the statistic in each year (sample) and the covariance of the statistic between samples.

1Sampling: Design and Analysis, 2nd Edition, Sharon L. Lohr (2010).

Data collection

The annual SILC survey is the main data source for SILC. In response to growing concerns related to community transmission of COVID-19, the CSO suspended all household survey fieldwork activities in mid-March 2020. SILC information before the onset of COVID-19 was collected from household members (16 years and older) by CSO interviewers, using Computer-Assisted Personal Interview (CAPI) in the respondents' homes. In March 2020 the CSO developed a SILC data collection instrument suitable for conducting SILC longitudinal interviews by telephone (Computer-Assisted Telephone Interview (CATI)). SILC 2022 data was collected via both CAPI and CATI from January to July 2022.

In addition, CSO uses primary micro data sources in its statistical programs to complement or replace survey data, to make its statistical operations more efficient or to create new insights or products. These data enable CSO to fill information needs about the Irish society, economy and environment, reduce response burden and costs imposed by surveys, and improve data quality and timeliness. All data obtained by CSO are used solely for statistical purposes.

The primary micro data sources are the Department of Social Protection (DSP) social welfare data, Office of the Revenue Commissioners’ Income Tax Form 11 and PAYE Income data, Department of Agriculture, Food and the Marine Animal Identification and Movement Data, Student Universal Support Ireland Grant Application and Payment Data, Local Authority HAP Shared Services Centre Housing Assistance Payments and the Residential Tenancies Board Rent Data.  The CSO continues to work with these sources to ensure good quality data is available on a timely basis. 

Classifications and Indicators

Principal Economic Status

From 2020 the question on Principal Economic Status was standardised under Regulation (EU) 2019/1700.  The categories are:

  • Employed
  • Unemployed
  • Retired
  • Unable to work due to long-standing health problems
  • Student, pupil
  • Fulfilling domestic tasks

Highest Level of Education Completed

From 2020, the highest level of education achieved is mapped using the International Standard Classification of Education (ISCED 2011) coding system and categorised as follows:

ISCED codeHighest Level of Education Classification
000 Less than primary education Primary or below
100 Primary education
200 Lower secondary education Lower secondary (including transition year)
300 Upper secondary education (not further specified) Upper secondary
343 Level completion, without direct access to tertiary education
300 Upper secondary education (not further specified)
344 Level completion, with direct access to tertiary education
300 Upper secondary education (not further specified)
450 Vocational education Post leaving certificate
400 Post-secondary non-tertiary education (not further specified)
500 Short cycle tertiary Third level non-degree
600 Bachelor or equivalent Third level degree or higher
700 Master or equivalent
800 Doctorate or equivalent

Household composition

For the purposes of deriving household composition, a child was defined as any member of the household aged 17 or under. Households were analysed as a whole, regardless of the number of family units within the household. The categories of household composition are:

  • 1 adult aged 65+
  • 1 adult aged <65
  • 2 adults at least 1 aged 65+
  • 2 adults, both aged <65
  • 3 or more adults
  • 1 adult, with children aged under 18
  • 2 adults with 1-3 children aged under 18
  • Other households with children aged under 18

Number of Persons at Work in the Household

The number of persons at work in the household is the number of persons that described their Principal Economic Status as Employed.

Tenure status

Tenure status refers to the nature of the accommodation in which the household resides. The status is provided by the respondent during the interview and responses are classified into the following two categories:

  • Owner-occupied
  • Rented or rent free

Urban/rural location

From 2020 onwards, areas are now classified as Urban or Rural based on the following area populations derived from Census of Population 2016:

Urban

  • Population >100,000
  • Population 50,000 – 99,999
  • Population 20,000 – 49,999
  • Population 10,000 – 19,999
  • Population 5,000 – 9,999
  • Population 1,500 – 4,999

Rural

  • Population 199 – 1,499
  • Rural areas in counties

Regions

The regional classifications in this release are based on the NUTS (Nomenclature of Territorial Units) classification used by Eurostat. The NUTS boundaries were amended on 21st November 2016 under Regulation (EC) No.2066/2016 and took effect from 1st January 2018. Results are presented at NUTS 2 level.  See Information Note for Data Users: revision to the Irish NUTS 2 and NUTS 3 Regions.

Deprivation rate

Households that are excluded and marginalised from consuming goods and services which are considered the norm for other people in society, due to an inability to afford them, are considered to be deprived. The identification of the marginalised or deprived is currently achieved on the basis of a set of eleven basic deprivation indicators:

  1. Two pairs of strong shoes
  2. A warm waterproof overcoat
  3. Buy new (not second-hand) clothes
  4. Eat meal with meat, chicken, fish (or vegetarian equivalent) every second day
  5. Have a roast joint or its equivalent once a week
  6. Had to go without heating during the last year through lack of money
  7. Keep the home adequately warm
  8. Buy presents for family or friends at least once a year
  9. Replace any worn out furniture
  10. Have family or friends for a drink or meal once a month
  11. Have a morning, afternoon or evening out in the last fortnight for entertainment

Individuals who experience two or more of the eleven listed items are considered to be experiencing enforced deprivation. This is the basis for calculating the deprivation rate.

Eurostat EU-SILC Estimates

Estimates produced from SILC data by the CSO are based on national definitions of income, equivalence scale, deprivation etc.  These are not directly comparable with EU-SILC estimates produced on the Eurostat website.  See the Eurostat webpage on Income and Living Conditions for further details.

Acknowledgement

The Central Statistics Office wishes to thank the participating households for their co-operation in agreeing to take part in the SILC survey and for facilitating the collection of the relevant data.

For further information on this release:

E-mail ICW@CSO.ie or contact Kathryn Foskin (+353) 21 453 5302, or Eva O’Regan (+353) 21 453 5243.