Sensory thresholds are ill-deﬁned in theory (Lawless and Heymann 1998; Morrison 1982).
A good determination requires hundreds of comparisons with a control and results do not
reproduce well (Brown et al. 1978; Marin, Acree, and Barnard 1988; Stevens, Cain, and
Burke 1988). Published group thresholds (Fazzalari 1978; Van Gemert and Nettenbreijer
1984; Devos et al. 1990) vary by a factor of 100 for quinine sulfate in water and by much
more in complex systems. Swets (1964) doubts even the existence of a sensory threshold. A
ﬁrst reaction is that it is futile to invest time and money in threshold studies; however,
in situations such as those described in the next paragraph, the threshold approach is still
the best available.