Figure 1. This illustration depicts mean vs. individual differences for hangover intensity in a randomized controlled crossover trial. Hangover severity is expressed as the ratio of acute hangover scale rating (AHS) and maximal breath alcohol concentration (BrAC) in accordance to type and order of alcoholic beverage consumption. For all groups, AHS/BrAC ratios are given for each individual on both study day participations (magenta), e.g. study group 1 on study day 1: beer first, then wine; vs. study day 2 (crossover for the same individual): wine first, then beer, etc. Mean differences and 95% confidence intervals are shown on the right (black).
Remarkably, while mean differences are small with narrow 95% CI, a considerable number of individuals demonstrated a strikingly different hangover response when comparing the two study participations. Because this individual effect is bi-directional, the result is an overall small mean difference for the cohort as a whole..