Actual and Perceived Units of Alcohol in a Self-Defined "Usual Glass" of Alcoholic Drinks in England

Background Several studies have found participants pour more than 1 standard drink or unit as their usual glass. This is the first study to measure actual and perceived amounts of alcohol in a self-defined usual glass of wines and spirits in the general population. Methods Participants were a convenience sample of adults who drink alcohol or who pour drinks for other people (n=283, 54% women) at 6 sites in South East England. The survey was face to face and comprised a self-completion questionnaire and pouring task. Estimation accuracy, categorised as correct (+/- 0.5units), underestimate (>0.5units), or overestimate (>0.5units) was the main outcome. Results The mean number of units poured was 1.90 (SD 0.80; n=264) for wine and 1.93 (SD 0.78; n=201) for spirits. The amount of alcohol in a self-defined usual glass was estimated in 440 glasses (248 wine and 192 spirits). Overestimation took place in 42% glasses of spirit poured and 29% glasses of wine poured, and underestimation in 17 and 19%, respectively. Multinomial logistic regression found volume poured to be significantly associated with underestimating both wines and spirits, and additionally for wine only, belonging to a non-white ethnic group and being unemployed or retired. Not having a university degree was significantly associated with overestimating both drink types. Conclusions This study is the first in the general population and did not identify systematic underestimation of the amount of alcohol in a self-defined usual glass. Underestimation is significantly associated with volume poured for both drink types; therefore, advocating pouring smaller glasses could reduce underestimation of alcohol consumption.