One explanation for the underestimation of the amount of alcohol consumed is the difficulty in accurately estimating the alcohol volume of drinks.
For example, Gill and O’May (2007)found misunderstandings regarding unit values
were influenced by the different size and shape of glasses. In their study, a sample of 297 people were presented with a range of different sizes of glass sold as ‘small’ and asked to select what they estimated would provide a one unit measure of alcohol.
The sample chose glasses with capacities varying between 250mL and 360mL, which is more than double the recommended small glass size of 125mL (DH 2007). Consumption of alcohol based on such decisions would result in individuals consuming double the recommended
units of alcohol.