Thresholds for three odorants were obtained for 100 judges. 1,8-Cineole, isobutyraldehyde and 尾-damascenone were tested in accordance with the data collection procedure of ASTM E679. Initially, thresholds were estimated by the ASTM-based method with four different stopping rules. These estimates were subsequently compared against thresholds estimated by an alternative method; that of fitting psychometric functions. The theoretical grounds on which the latter method is based, and the clear connection between performance and stimulus concentration that it demonstrates, provide a thorough comparison of both conventionally-defined thresholds and method-independent thresholds against the ASTM-based method.
Comparisons between the thresholds resulting from the various estimation methods demonstrated (1) the choice of stopping rule had a significant effect on threshold estimates (P < .05); (2) each stopping rule measured a specific level of performance, indicating a useful correspondence between the stopping rules and the psychometric function. These findings provide supporting evidence for the continued use of the ASTM-based method; albeit, if the stopping rule is standardized. One of the tested stopping rules (Stop 3) provided threshold estimates consistent with those estimated for d鈥?#xA0;= 1.0, and is therefore recommended for future research.