BACKGROUND AND AIM: Sampling frames used by population surveys may result in an underestimation of smoking prevalence as those not residing in households, also known as the 'hidden population', are not sampled. This includes people living in care and residential homes, populations experiencing homelessness, as well as those who have an "absent or "temporary" household status (e.g. bed and breakfasts, unsupported temporary accommodation, and those in prison). This study estimated the impact of including these populations on smoking prevalence in the UK.

METHODS: Data from UK government reports and published studies were used to derive estimates of the size of the hidden populations and rates of smoking within these populations. The impact of including the 'hidden population' on overall smoking prevalence in the UK was estimated, along with a correction factor to account for their exclusion.

RESULTS: The hidden population over the age of 18 was estimated to be around 1.9 million with a smoking prevalence of between 58% and 66%. Accounting for this hidden population in smoking prevalence surveys may inflate estimates by a correction factor of 1.12 to 1.14. This means smoking prevalence in 2022 would increase from a reported 12.9% to an adjusted range of 14.5-14.8%.

CONCLUSION: The absence of the 'hidden population' from smoking prevalence surveys leads to underestimation of smoking rates in the UK. Based on the estimated correction factor, and assuming smoking continues to decline at current rates, achieving the UK government's target of <5% by 2030 would be substantially delayed.