The Scottish Health Survey 2024 - Volume 2: Technical Report

1.2   Sample design

1.2.1    Requirements

The sample specification for the 2024 SHeS was designed by the Scottish Government. The design was coordinated with the designs for the Scottish Household Survey (SHS) and the Scottish Crime and Justice Survey (SCJS) to improve survey efficiency and to allow the samples of the three surveys to be pooled for further analysis[i].

There were two elements to the SHeS sample in 2024:

1. Main sample - to allow annual reporting of Scotland level results and results at Health Board and local authority level for adults using the 2021, 2022, 2023 and 2024 data combined. This required an annual interview target of 5,112 adults for Scotland as a whole and a minimum target of 125 for each local authority. There were additional targets of 1,000 adults to complete each biological measure and 1,026 child interviews.
2. Child boost sample – overall there was a requirement for 2,031 child interviews for Scotland. As the main sample was only expected to yield 1,026 child interviews, a further 1,005 interviews were required from a separate boost sample.

1.2.2    Sample design and assumptions

In 2024, the sample utilised a two-stage clustered sample design, with intermediate geographies randomly selected at the first stage and address points at the second stage. With the exception of Orkney, Shetland and Na h-Eileanan Siar councils, the sample was clustered by intermediate geographies (IG) with one quarter of IGs selected for each year of fieldwork. In Orkney, Shetland and Na h-Eileanan Siar the sample was clustered by data zone.

1.2.3    Main sample

As stated above, the annual target sample size for Scotland was 5,112 adults with a minimum local authority target sample size of 125 adults. These sample sizes were the minimum required to allow effective reporting of Scotland-level results annually and Health Board and local authority results with four years of data combined. An iterative approach was taken to efficiently allocate the sample across all Health Boards and local authorities. For the first iteration, 4,000 adult interviews were allocated across local authorities in proportion to the adult population. Any local authorities allocated fewer than 125 adult interviews had their allocation increased to 125.

The remaining sample was then allocated over the remaining local authorities. Where allocations were not whole numbers the number was rounded up. This resulted in a total target of 5,112 adult interviews. The results of the allocation are shown in Table 1.

Table 1: SHeS target annual adult interviews, 2024, by Health Board

To allow for reporting at local authority level over a four-year period (2021/2022/2023/2024) and coordination with the sample selection of the SHS and SCJS, the required sample sizes were set at local authority level. This was done by allocating the target Health Board samples to local authorities proportionate to population.

The number of addresses selected in order to provide the target number of interviews was calculated by:

1. Estimating the number of productive adult interviews per co-operating household. Considering response data from previous years, it was estimated that there would be an average of 1.5 interviews per co-operating households in each local authority.
2. Allocation of the target interviews and associated estimate of co-operating households to local authority strata proportionate to population.
3. The response rate assumptions for local authorities for 2024 were then estimated based on the variation across local authorities in response to the 2018, 2019 and 2023 face-to-face surveys. 
4. The final step was to estimate the level of ineligible addresses. As for previous survey years, the estimates were calculated at local authority level and based on the average level of ineligible addresses from previous years of SHeS, SHS and the SCJS.

Table 2 shows the number of selected addresses used for the main sample in 2024.

Table 2: Selected addresses issued by strata in 2024 – main sample

1.2.4    Child boost sample

For the 2024 survey, 2,031 child interviews were required. It was estimated that the main sample would provide 1,026 child interviews, therefore, to reach the target number of child interviews, a child boost sample was required to yield a further 1,005 interviews.

The child boost sample was linked to health records via the Community Health Index (CHI) to identify households with children. This significantly improved the sample efficiency and response levels. This linkage was carried out by the CHI Linkage (CHILi) Indexing Team at Public Health Scotland.

Table 3: Target annual child interviews, 2024, by Health Board