Weighting LFS survey data to get population based estimates
The LFS collects data on a sample of the population. To convert this information to give estimates for the population, the sample data is weighted. Each case is given a weight which can be thought of as the number of people that case represents. This weighting factor takes account of differential non-response among different sub-groups in the population. This weighting procedure involves grossing data to sub-regional population estimates and then adjusting for the estimated age and sex composition, by region.
These weighting factors are provided to HSE by the Office for National Statistics (ONS) along with the LFS dataset itself.
The Coronavirus (COVID-19) had an impact on the achieved LFS sample in 2020 quarter one; social distancing measures were implemented towards the end of the quarter leading to a decline in response rates as face-to-face interviews moved to telephone interviews (see LFS Performance and Quality Monitoring Reports: January to March 2020 for more details). The uneven distribution of the achieved sample across the quarter has been accounted for with methodological changes to the LFS weights. The overall achieved sample for 2020 quarter one (77,903 individuals in 33,329 households) was about 11% lower than the achieved sample for 2019 quarter one (87,417 individuals in 37,167 households).
Whilst the ONS provided weighting factors take account of non-response to the LFS itself, they do not account for non-response to individual questions within it, in particular the Workplace Injury and SWI survey modules' screening questions. In 2001/02, just over 5% of eligible respondents did not respond to either or both of the screening questions within the workplace injury and SWI survey modules (most of whom were unavailable for interview); in recent years this has increased to over 15%. Since this non-response is not accounted for in the standard LFS weighting procedure, an adjustment is made to the weights to take account of this non-response and provide more reliable estimates.
For the non-response adjustments a procedure is implemented to identify demographic and employment-related variables and codes that best describe the pattern of response to the ill health and workplace injury screening questions. The procedure progressively splits the sample into groups between which the response rates differ to a statistically significant extent. The adjustment for non-response then involves multiplying the standard LFS weights by the reciprocal of the response rate within each of the groups identified.
ONS revise the LFS weights (estimates for the population) following a census. A new policy was introduced in 2016, to revise weights annually rather than periodically between censuses, providing users with the most up to date estimates. The frequency was changed to every other year from 2018. Revisions are generally made to the previous three years. HSE assess the impact on the work-related illness and workplace injury published estimates, and a judgement about the need to revise is made. Any subsequent revisions made are noted in the revision log.