This is the smallest detectable change. Weber concluded the JND was a constant fraction of the stimulus e.g. weight is about 1 in 40, and brightness of white light is about 1 in 60. Fechner suggested the number of JNDs from zero to the magnitude of a stimulus is a measure of its sensory intensity. An absolute threshold is the JND from zero to detectable. A difference threshold is the JND from a detectable level to a change in that level.
Thresholds are somewhat variable, in practice.
Or it can be MISSed. We might be biased in our responses. A response towards saying yes, a signal would be encouraged by demanding few misses. Sensitivity can be measured from a graph plotting percentage of hits against percentage of false alarms . On this graph, if false alarm percentage increases equally as percentage of hits increases, the observer is behaving at random.