The Scottish Health Survey 2024 - Volume 2: Technical Report

 2.1 General survey definitions

Age standardisation

Age standardisation has been used in some analysis in order to enable groups to be compared after adjusting for the effects of any differences in their age distributions.

When different sub-groups are compared in respect of a variable on which age has an important influence, any differences in age distributions between these sub-groups are likely to affect the observed differences in the proportions of interest.

Age standardisation was carried out, using the direct standardisation method. The standard population to which the age distribution of sub-groups was adjusted was the mid-2022 population estimates for Scotland. All age standardisation has been undertaken separately within each sex.

The age-standardised proportion p' was calculated as follows, where pi is the age specific proportion in age group i and Ni  is the standard population size in age group i:

Therefore p' can be viewed as a weighted mean of pi using the weights Ni . Age standardisation was carried out using the age groups: 16-24, 25-34, 35-44, 45-54, 55-64, 65-74 and 75 and over. The variance of the standardised proportion can be estimated by:

Confidence intervals

The tables presented in the main report for 2024 contain 95% confidence intervals (CIs). These represent the upper and lower values between which it is statistically probable that a percentage or mean reported will fall between, showing the variation around these results. Confidence intervals help to interpret how precise a result is.

Clusters

A cluster sampling approach was used whereby addresses across Scotland were divided into smaller groups (or clusters) before then using a random sample selection approach to select a sample to approach for the survey.

Household

A household was defined as one person or a group of people who have the accommodation as their only or main residence and who either share at least one meal a day or share the living accommodation.

Household Reference Person (HRP)

The HRP is defined as the householder (a person in whose name the property is owned or rented) with the highest income. If there is more than one householder and they have equal income, then the household reference person is the oldest.

Local Authority

32 elected councils make up the Scottish Local Government, which are referred to as Local Authorities.

Mean

Most means in this report are arithmetic means (the sum of the values for cases divided by the number of cases). 

NHS Health Board

The National Health Service (NHS) in Scotland is divided up into 14 geographically based local NHS Boards and a number of National Special Health Boards. Health Boards in this report refers to the 14 local NHS Boards.

Percentile

The value of a distribution which partitions the cases into groups of a specified size. For example, the 20th percentile is the value of the distribution where 20 percent of the cases have values below the 20th percentile and 80 percent have values above it. The 50th percentile is the median.

Primary sampling unit

A primary sampling unit (PSU) is the first stage of the sampling process. All primary sampling units (data zones on the islands, intermediate geographies elsewhere) in Scotland were randomly allocated to one of four sets. One of these sets is used in each year of fieldwork. This means that the sample is drawn from one quarter of PSUs each year.

P-value

A p value is the probability of the observed result occurring due to chance alone. A p value of less than 5% is conventionally taken to indicate a statistically significant result (p<0.05). It should be noted that the p value is dependent on the sample size, so that with large samples differences or associations which are very small may still be statistically significant. Results should therefore be assessed on the magnitude of the differences or associations as well as on the p value itself. The p values used in the analysis for this report take into account the clustered sampling design of the survey.

Quintile

Quintiles are percentiles which divide a distribution into fifths, i.e., the 20th, 40th, 60th and 80th percentiles.

Sample

The survey sample represent a proportion of the whole population living in Scotland. It is not possible to survey every household in Scotland, therefore, a proportion of addresses are selected, or sampled, each year in such a way as to be as representative of the whole population as possible. See chapter 1 of the technical report for more information on the sampling approach used.

Scottish Index of Multiple Deprivation (SIMD)

The SIMD is the Scottish Government’s official measure of area based multiple deprivation. It is based on 37 indicators across 7 individual domains of current income, employment, housing, health, education, skills and training and geographic access to services and telecommunications. SIMD is calculated at data zone level, enabling small pockets of deprivation to be identified. The data zones are ranked from most deprived (1) to least deprived (6,505) on the overall SIMD index. The result is a comprehensive picture of relative area deprivation across Scotland.

This report uses:

• SIMD 2006 for 2008 data
• SIMD 2009 for 2009-2011 data
• SIMD 2012 for 2012-2015 data
• SIMD 2016 for 2016-2018 data
• SIMD 2020 for 2019 and 2021- 2024 data (see Scottish Index of Multiple Deprivation 2020).

Breakdowns by SIMD for earlier years are available in the SHeS dashboard. 

Self-completion booklet

A portion of the survey questions included in SHeS are asked via a self-completion booklet, including some questions that could be viewed as more sensitive. These paper booklets were handed to respondents who completed them without interviewer assistance (unless this was requested). The booklets were returned to the interviewer before leaving the household.

Significance testing

Where differences in relation to a particular outcome between two subgroups, such as males and females, are highlighted in the main report, the differences can be considered statistically significant, unless otherwise stated.

Statistical significance is calculated using logistic regression to provide a p-value based on a two-tailed significance test. One-tailed tests are used when the difference can only be in one direction. Two-tailed tests should always be used when the difference can theoretically be in either direction. For example, even though previous research has shown a higher prevalence of hazardous levels of alcohol consumption among males than among females, and we may expect this to be true in the most recent survey, a two-tailed test is used to confirm the difference.

Unweighted bases

The unweighted bases presented in the report tables provide the number of individuals upon which the data in the table is based. This is the number of people that were interviewed as part of SHeS and provided a valid answer to the particular question or set of questions. The unweighted bases show the number of people interviewed in various subgroups including sex, age and Scottish Index of Multiple Deprivation.

Weighting

While SHeS is designed to have as representative a sample as possible, it is not possible to obtain a fully representative sample as participation is on a voluntary basis. Therefore, statistical adjustments are undertaken to ensure that the data is representative of the population of Scotland, according to the most up-to-date population estimates. For more information on the weighting approach used, see chapter 1 of the technical report.

Weighted bases

The weighted bases are adjusted versions of the unweighted bases which involves calculating a weight for each individual so that their representation in the sample reflects their representation in the general population of Scotland living in private households. Categories within the table can be combined by using the weighted bases to calculate weighted averages of the relevant categories.