Experimental design. a, The study involved four identical visits. 412 subjects completed the first two visits. Of those, the 21 with the lowest general olfactory acuity were excluded (subjects in red). 56 subjects were invited for visits three and four (subjects in green). b, There was a spacing of approximately seven days between visits one and two and between visits three and four. There was a spacing of more than one year between visit two and three. c, During each visit, the intensity and pleasantness of 159 stimuli was rated. After the subjects rated 53 stimuli they assigned descriptors to spearmint oil, pentadecalactone, and the solvent propylene glycol. After they rated 106 stimuli, they assigned descriptors to vanillin and androstenone. After the intensity and pleasantness rating the thresholds for pentadecalactone, isovaleric acid, and vanillin were measured. See the “odours and sequence of stimuli” tab of Additional file 1 for the complete sequence of odours and concentrations used in the study.