This article throws light upon the top three methods of studying the problems of threshold and equivalence. The methods are: 1. Method of Average Error 2. Method of Minimal Changes or Limits 3. The Constant Stimulus Method.
1. Method of Average Error:
The unique feature of this method among the other psychophysical methods is that the subject manipulates the variable stimulus (SV) in relation to a standard stimulus (SS). In a typical experiment (like in Muller-Lyer experiment) where the method of average error is employed, the value of SV is always too higher or lower than the SS so that the subject perceives the difference between the SV and SS.
The subject then decreases or increases the value of SV continuously in the same direction till he perceives the SV as equivalent to SS. This marks the end of one trial. At the end of each trial, the error of the difference is noted down and the experiment is repeated a number of times.
Finally, the average error is worked out. This explains the name of the method. This method is also known by other names such as the method of reproduction because the subject attempts to reproduce the standard stimulus, the method of adjustment because the subject adjusts the value of SV equivalent to the SS and the method of equivalent stimuli because the experimenter wants to study the subject’s phenomenal equivalence of a standard stimulus.
2. Method of Minimal Changes or Limits:
The method of limits is another important psychophysical method. The application of this method has been illustrated in the experiments on limits of auditory sensation and in the experiments on two-point threshold.
Here the essence of the method lies in approaching in gradual steps a particular experience of a subject from a point of no experience and approaching the absence of the same experience from a point of definite experience.
These two are called the ascending and descending series. For example, in the experiment on two-point threshold, if we start with a distance between the two points (which is definitely perceived as one) and increase the distance in small steps until the subject reports an experience of two-point stimulation, it will be an ascending series.
On the other hand, if we start with a definite two-point sensation and gradually reduce the distance between the two points until the subject reports one-point stimulation, this will be a descending series. Between the above two psychophysical methods the method of average error has certain advantages over the method of limits.
Chief among them are:
(a) The subject is actively involved in the experiment when the method of average error is employed, and
(b) It is easier to change the variable stimulus continuously as in the case of the method of average error rather than changing the SV in equal and small steps as in the method of limits.
3. The Constant Stimulus Method:
Constant Stimulus method is generally regarded as the most accurate of all the psychophysical methods but this is too cumbersome for the experimenter as well as to the subject. If the experimenter and the subject can put up with its procedural wrangles, then this method yields the best possible estimates and controls almost all the errors that enter into the experiment while the methods of average error and limits are employed.
The possible errors that might enter the constant methods are the time order error and the errors caused by the fatigue on the part of the subject and also, possibly, on the part of the experimenter. However, these two errors can easily be controlled.
The time order error can be controlled by presenting the SS and SV in one order on half of the trials and in the reverse order on the remaining trials. The fatigue can be controlled by introducing optimum rest pauses whenever needed during experimentation.
The typical method followed is as follows. The experimenter selects certain fixed values of variable stimulus in such a manner that half of them are greater than the SS value and the remaining half less than the SS value.
Each variable stimulus is presented along with or in succession to SS a large number of times, say ranging from 50 to 200, by the experimenter. However, the different variable stimuli are presented in a prearranged random order by the experimenter.
On each occasion (trial) the subject has to report whether the variable stimulus is greater or lesser than the SS when a two-category judgement is employed and the third category of judgement is ‘doubtful’ when three-category judgement is employed.
However, for finding the absolute limen, each stimulus is presented alone and the subject judges either the presence or absence of the desired experience; for example, ‘two’ when studying the two-point threshold.
The former procedure is known as the method of constant stimulus difference and the latter as the method of constant stimuli. In either case the limens are computed from the percentages or proportions of judgements of different kinds for each stimulus.
As a result of such experiments a few psycho physical laws also were evolved. The two well-known and classical laws are Weber’s law and Fechner’s law.
This law, formulated by E.H. Weber, was concerned with the estimation of the differential limen or the Just Noticeable Difference (JND). Briefly the law asserted that the differential limen bears a constant ratio to the original stimulus.
Mathematically, it can be stated as:
Delta S/S = C
Where Delta S means stimulus increment necessary to produce, a Just Noticeable Difference.
S = Original stimulus
C = Constant
According to this law, if original stimulus of hundred units requires an increment of 5 units to produce a Just Noticeable Difference, then a stimulus of two hundred units will require an increment of 10 units to produce a Just Noticeable Difference, a stimulus of 300 units requires 15 units and so on.
T. Fechner, on the other hand, was interested not only in measuring differential limens but was interested in predicting the total sensation or response from a known value of the stimulus. Fechner’s Law stated that the amount of sensation varies as the logarithm of the stimulus.
According to this law if a stimulus of 100 units produces a sensation of two degrees, a stimulus of 1000 units will produce a sensation of three degrees. This increase in sensation is slower than the increase in the stimulus. In fact, sensation increases arithmetically while stimulus increases geometrically. Weber’s Law and Fechner’s Law are often together referred to as the Weber-Fechner Law.
Thurstone’s Law of Comparative Judgement:
Classical psychophysics was essentially involved in establishing quantitative relationships between stimuli and responses and was not very useful in the direct measurement of psychological processes. The work of Thurstone, to a great extent, proved useful in enabling psychologists to arrive at measurements of response directly and not in terms of stimulus values.
He formulated the law of comparative judgement which provided a rationale for measurement of responses and thus, ushered in what is known as modern psychophysics. The explanation of Thurstone’s law is rather complicated and perhaps not required here.