The ability of human beings to discriminate signals and the length of time taken to arrive at different forms of decisions are considered in this section. Finally, we dis­cuss the way in which ageing and training can affect decision making.

10.4.1 Discrimination of Signals

If there are 10 lamps lit in a room, and one more is lit, the increase is perceived as being considerably greater than if there had been 100 lamps lit in the room and one more had been lit. The same is true for weight; if a 1 kg weight is added to another 1 kg weight, the weight change is felt as considerably greater than if a 1 kg weight is added to 10 kg.

Two researchers, Weber and Fechner (reported in Stevens, 1975), developed a law based on these perceptions. They stated that the difference between stimuli that were just perceptible (JND, just noticeable difference) were always of the same psy­chological strength. A certain psychological JND should thus cause the same psy­chological effect as another JND under different conditions. If the JND in weight increase above 1 kg is 1 g and 100 kg is 100 g, the psychological effect of the first increase (1 g) should be as great as the second increase (100 g). In practice, this means that a small increase in a low-intensity stimulus creates the psychological experience as a considerably greater increase in a high-intensity stimulus. The law produced by Weber and Fechner from this background can be expressed thus:

n = K log I (10.2)

where I is the signal intensity and n is the number of JNDs and K is a constant that depends on the type of signal. In other words, n is a form of index of the psychologi­cal perception of the signal.

The psychologist, Stevens, designed another index that is somewhat easier to use. According to Stevens (1975), the relationship between the psychological percep­tion P and the intensity of the signal can be expressed thus:

P = Lla (10.3)

L and a are two coefficients that depend on the characteristics of the signal. As Fig­ure 10.8 shows, L represents the point at which the curve meets the vertical axis, and a is the slope of the line.

Stevens’ law has been used for describing the psychological perception of a num­ber of different factors such as light signals, sound signals, and lifting of weights. Some examples of the values of the exponent a for various types of tasks are shown in Table 10.8.