Within-individual variability of odour perception. The average absolute change in intensity and pleasantness ratings of fifteen stimuli were measured for 56 subjects twice on each of four visits. a, The absolute intensity and pleasantness changes were calculated by averaging the absolute difference between two ratings performed by the same subject. For the 30 min measure, the first and second ratings were compared for all four visits. For the 7 days measure, the second rating of the first visit and the first rating of the second visit and the second rating of the third and first rating of the fourth visit were compared. For the 1 year measure, the second rating of the second visit and first rating of the third visit were compared. There was about one week between visits one and two and between visits three and four. Visits two and three were more than one year apart. b, The average absolute change in intensity (left) and pleasantness (right) ratings for each stimulus is shown for three time intervals between ratings: 30 minutes, one week, and more than one year (519 days on average). An absolute change of “1” is equivalent to a change of one step on the rating scale (for example from “very weak” to “extremely weak”). Data are plotted as mean±standard deviation of the mean (S.D.). Odour names are labelled according to odour concentration: grey: low; black: high.