The census aims to capture details of the whole population of Scotland. However, it is expected that during the census some people and households will be missed.
Estimation and Adjustment is used to find out how many households and people we collected information from during census. Where evidence suggests there is an undercount, records are added to give a census dataset for the whole population of Scotland.
The estimation process produces overall population and household estimates.
It requires clean and complete datasets for both the census and from a second survey – the Census Coverage Survey (CCS). Having this second set of collected data is key to being able to generate population estimates.
The people and households in the census and CCS are linked to work out:
- how many were captured in both the census and the CCS
- how many were only found in the census
- how many were only found in the CCS
Using standard statistical methodology called Dual System Estimation (DSE), the results of this matching can be used to estimate how many households or people were missed by both the census and the CCS.
There are also corrections made to the estimates to account for people who were counted more than once, or were counted in the wrong place.
The adjustment process creates new records for the missed population. When combined with the Estimation process, this gives a census dataset for the whole population.
Once the final estimates have been agreed, the adjustment process creates donor records. These donor records represent the people and households that were missed in the census. The final census dataset should then match, as closely as possible, the estimates we have produced during the estimation process.
There are a number of ways to do this:
- Add people to existing households
- Add people to existing communal establishments
- Create new households in a ‘space’ we already know about (for example a known address that is an occupied property from which we received no response)
- Create new households in a ‘space’ that we don’t have an address for but allocate them a real postcode so we know where they are
This is a complex process that requires a combination of different statistical methodologies. They are used to calculate, for each census record we received, the probability that a person or household with those characteristics would be missed from the census.
This information is then used to pick a number of existing person records to use as donors and key characteristics from these donors are used to create new person records – this is the unit-level imputation process.
More information on Estimation and Adjustment can be found in the following External Methodology Assurance Panel documents:
- PMP001: Estimation and Adjustment Methodology
- PMP009: Estimation Areas - Geographical grouping for the stratification of population estimates
- PMP010: Census to Census Coverage Survey (CCS) linking
- PMP012: Census Coverage Survey Household Links
- PMP016: Estimation - Household Bias Adjustment
- PMP017: Census to census linking for estimating overcoverage
Statistical Methodology Rehearsal 2020
From April to June 2020, National Records of Scotland (NRS) carried out a rehearsal of several statistical methodologies for Scotland’s Census 2022. The Estimation and Adjustment methodologies were tested during this rehearsal.
The evaluation reports from this rehearsal can be found on the Statistical Methodology Rehearsal 2020 web page, which will be published in September 2020.
Statistical Quality Assurance
NRS has published a Statistical Quality Assurance Strategy which provides more information on how the Estimation and Adjustment process, and others, will be quality assured.
NRS aims to harmonise statistical methodologies with other UK census offices as much as possible. We share ideas and provide feedback on methodologies through harmonisation working groups with the Office of National Statistics (ONS) and the Northern Ireland Statistics and Research Agency (NISRA).
For more information on how NRS work with the other UK censuses to harmonise our statistical methodology for UK Data Users, as well as share best practice and lessons learned, please see the UK Census Data tab of this website.
In February 2020, NRS ran Statistical Methodology Stakeholder events aimed at the general public and data users. These events gave attendees a high level overview of what happens to census data from when NRS receives census responses through to producing the outputs.
During these events, we sought feedback to help to further develop our plans to ensure the highest quality of outputs for our users. Slides from the event are published on the event page of the Scotland’s Census 2022 website.
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