How to use the information in this report
All results shown in this report are statistically significant at the 95 per cent level (see Annex 3 of the main annual report for a more detailed explanation).
Although the SHS sample is chosen at random, the people who take part in the survey will not necessarily be a representative cross-section of the population. Like all sample surveys, the results of the SHS are estimates for the whole population and these results might vary from the true values in the population. As a result, all survey estimates have an associated confidence interval (within which the ‘true’ proportion of the whole population is likely to lie), usually expressed as ±x per cent. As a general rule of thumb, the larger the sample size for a given question, the smaller the confidence interval around that result will be, thus making it easier to detect real change year-on-year and differences between sub-groups.
Conventionally, the confidence level is set at 95 per cent. This means if the survey were to be run multiple times on the same population in the same year, we would expect that the achieved result to lie within the confidence interval in 95 out of 100 surveys.
Where sample sizes are small or comparisons are made between sub-groups of the sample, the sampling error needs to be taken into account. There are formulae to calculate whether differences are statistically significant (i.e. they are unlikely to have occurred by chance). For guidance on this, please see Annex 3 of the main annual report which provides a simple way to calculate whether differences are significant, as well as explanation on statistical significance and on how confidence intervals can be interpreted.
Glossary and annexes
For definitions of the key terms, please see our Glossary. For further guidance on assessing confidence intervals and the statistical significance of the results please see the annexes to the main report published on our website.
How data is displayed in charts
All charts have a numerical base showing the population or population sub-group examined in it. While all results have been calculated using weighted data, the bases shown provide the un-weighted counts, which have been rounded to the nearest 10 to comply with statistical disclosure control principles and the Code of Practice for Official Statistics. It is therefore not possible to calculate how many respondents gave a certain answer based on the results and bases presented in the report.
Variations in Base Size
The SHS sample in 2017 was 10,680. However, some questions are asked of a reduced sample and the bases are correspondingly lower. While we try to keep non-responses to a minimum, bases do fluctuate slightly due to small amounts of missing information (for example income).
Email: Emma McCallum