This method can be also used for measuring the differential threshold.
The task of the observer is to examine pairs of stimuli and to judge which produces a bigger magnitude
- One of the two has a fixed value: STANDARD STIMULUS (St)
- The other one has a variable value, and is called COMPARISON STIMULUS (Co) and is changed from trial to trial, being sometimes greater than, sometimes smaller, and sometimes equal to the value of the standard stimulus.
Usually five, seven or nine values of comparison, separated by equal distances on the physical scale, are employed.
The values of the comparison are chosen so that the stimulus of greater magnitude is always judge as greater than the standard and the stimulus of less magnitude is always judge as smaller than the standard.
There are usually the same number of comparison stimuli above and below the standard stimulus and the subject report which has the greater sensory value.
Under ideal conditions standard and comparison stimuli would be presented together in space and time to permit optimal discriminability. Since this ideal is impossible in fact sensation occurring in the same time and initiated at the same receptive areas would blend together and become completely indiscriminable.
The two stimuli must be presented to different receptive areas at the same time, or to the same receptive area at different times.
The particular circumstances of an experiment usually determine whether the stimuli are presented simultaneously or successively and to the same or to different receptive areas.
Both the conditions may lead to certain errors and these have to be controlled.
- Space Errors
- If stimuli are presented to different receptive areas judgment may be affected by differences between the receptive area as well as differences between stimuli.
- To control the effect of space error, the standard stimulus may be presented on half of the trials to one receptor area, and on the half of the trials to the other receptor area.
- In this way the effect of spatial location would be neutralized.
- Time Errors
- To control the effects of time errors, the standard stimulus may be presented on half of the trials first and on half of the trials after the comparison.
- In this way the effect of temporal location would be neutralized.