METHOD OF AVERAGE ERROR By Navdeep Batth Lecturer D.A.V. College For Girls Yamuna Nagar

It is one of the oldest and most fundamental method. The aim of the method is to determine equal stimuli by active adjustment. Observer is provided with standard stimulus (Ss) Variable stimulus (Vs) is also provided. Variable stimulus is greater or less than standard stimulus Observer adjusts variable stimulus until it seems equivalent to standard stimulus. Number of such judgements are obtained from observer.

OTHER NAMES Method of reproduction Method of adjustment Method of equivalent stimuli

ORIGIN OF THE METHOD According to Titchner “ the method is free gift to psychophysics from the exact sciences of physics and astronomy.” Fechner introduced this method into psychophysics. Used for visual and tactual measurements.

AN EXPERIMENT OF MULLER LYER ILLUSSION Variable stimulus is the line with feathers or feather headed line Standard stimulus is a arrow headed line In half of the observations variable stimulus should be on observer’s left and on right in other half.

These two space arrangements are denoted as L and R. The trial should start with variable stimulus obviously too great so that the movement of the terminal arrow is inward. In other half variable stimulus variable stimulus is set obviously too small so that the movement is outward. This is denoted by I and O There are consequently four possible combinations of condition RO, RI, LO and LI.

Counterbalancing is followed within each set of trials. Experimenter should randomize the setting among large, medium and small starting length for variable stimulus.

DESIGN OF THE EXPERIMENT Sr.No RLRL IOOIIOOI Mean

RLRL design is used. This is used to control space error. IOOIIOOI design is used to control movement error.

If the mean of the judgment is greater than variable stimulus and difference is positive. This shows that feather headed line is overestimated. Arrow headed line has been underestimated. There are other constant errors.

CONSTANT ERROR OF SPACE Mean of all right trials – Mean of all left trials 2 Mean of all right trials = = Mean of all Left trials = = – = 0.49 = 0.24 cm 2 2

CONSTANT ERROR OF MOVEMENT Mean of all Inward trials - Mean of all outward trials 2 Mean of all Inward trials = = Mean of all Outward trials = = = – = = cm 2

PRACTICE/FATIGUE EFFECT Mean of first 50% trials – Mean of last 50% trials 2 Mean of first 50% trials = = Mean of last 50% trials = = = 9.65 – = = cm 2

POINT OF SUBJECTIVE EQUALITY- It is the point where participant/subject perceives arrow headed line equal to feather headed line PSE = Mean of all the trials 8 = = cm 8

CONSTANT ERROR PSE – Standard stimulus =11.76 – 15 = cm

DETERMINATION OF DL Probable error is calculated. Standard deviation from PSE value is measured. Standard deviation is calculated from PSE value. Standard deviation is multiplied with a constant value i.e

ADVANTAGES This method has practical advantage of economizing the time of both E and O. This method is useful when number of measurements are to be made in a limited time. This method is “most natural” of all methods. The observer participates actively in the judgement.

DISADVANTAGES Perceptual Vs Motor errors Constant time error Other uncontrolled errors

APPLICATIONS It applies in those cases where observer can manipulate the variable stimulus. Applied where variable stimulus is constantly variable. Most useful application in study of visual extent. It has been used in the study of visual intensities. Used in study of tonal attributes, pitch and intensities.

It has been adapted to the study of bodily movements. It can be adapted to certain problems in memory.

Method OF CONSTANT STIMULI It is most accurate and widely applicable method. They are employed in the measurements of stimulus limens, differential limen equal sense distances and equivalent stimuli. The difference between this method and method of constant stimuli is that the stimulus is presented in random and quasi random fashion. This eliminates error of habituation and error of anticipation.

OTHER NAMES METHOD OF CONSTANT STIMULUS DIFFERENCE- The same comparative stimulus are compared with standard stimulus. FREQUENCY METHOD – As the frequency of each stimulus value to be presented to the participant is same.

The experimenter presents the stimuli which lie in the transition zone. Experimenter uses the stimuli which lie in the range between those which can always be perceived and those which can never be perceived. Limited range is used because in each trial all the stimulus values have to be presented.

DETERMINATION OF AL Determine the range of stimulus values – The stimulus that is perceived just above zero percent of the trials. The one that is perceived little below hundred percent of the trials. This is selected as range to be employed.

Selection of stimulus values – Experimenter selects few stimulus values from the range. Some above and some below tentative AL. Spacing stimulus values at equal intervals – The stimulus values are placed at equal intervals on stimulus continuum.

Number of trials are determined – For the experiment 10 trials are determined. Number of trials should be determined before beginning with the experiment. Presentation of stimulus in random order – Stimulus values should be presented in random order. Various randomization techniques can be used.

Slip method. Random number tables. Flip of coin All stimulus values should be presented equal number of times.

DETERMINATION OF AL Let us take the ordinary two point tactual threshold. Region on the skin is selected on which the limen is to b determined. E makes some preliminary trials with compass points or asthesiometer. Transition zone is roughly determined. Some judgements are “two” and some are “one”.

Experimenter selects five stimuli such that the middle one is probably close to the limen. The smallest stimulus is likely to give report of “two” about 5% of the time. The largest stimulus to give report of “two” 95% of the time. The stimulus intervals are equal. Experimenter then applies the stimulus in random until each stimulus has been applied equal number of times.

The data from which the limen is to be computed consist of the proportion of the time each stimuli receives the judgment “two”.

HYPOTHTICAL DATA Stimulus (in cm) % (2) p% (1) p

THE INTERPOLATION METHOD- The two point limen is that separation of two points that yields 50% judgment of “two” and 50% judgment of “one”. In the data 50% judgment of 2 responses lie between 40% to 60%. AL is in between 3cm and 2.5 cm. AL is computed by difference between 3cm and 2.5cm.

THE PROPORTION METHOD – AL = sl+ (sh – sl) (.5 –pl) ph – pl Where, sl = stimulus immediately lower than the limen. sh = stimulus immediately higher than the limen. ph = proportion of “two” for the stimulus immediately higher than the limen pl = proportion of “two” for the stimulus immediately lower than the limen

AL = ( 3 – 2.5) (.5 –.4) = (0.5) (.1) 0.2 = = = 2.75 cm

DETERMINATION OF DL This requires the addition of a standard stimulus. On each trial a pair of stimuli and one of the variable stimuli are presented to subject. Subject judges whether one member of the pair is “greater than” or "less than” the other or middle category such as “equal” or “doubtful” Experiment begins with determining DL for lifted weights. Three category judgment is done.

The standard weight is of 50 gm. The variable weights range from 40 to 60 gm. Observer lifts the standard weight (50 gm) first and variable weight after that in half of the trials. In other half of the trials variable weight is lifted prior to the standard weight. He renders judgment always with respect to the second stimulus reporting it “greater”, “less” or “doubtful” as compared with the first.

DATA FROM LIFTED WEIGHT EXPERIMENT Stim ulus …20+= = =++= =+=== ====== ==-====== =----== = UT LT

Upper threshold (UT) UT = Sl + ( Sh – Sl) (.5 – Pl) ( Ph – Pl) Lower Threshold (LT) LT = Sl + ( Sh – Sl) (.5 – Pl) ( Ph – Pl)

IU = UT – LT DL = IU 2 PSE = UT + LT 2 CE = PSE – standard stimulus

ADVANTAGES It is a versatile method and has broad range of applicability. Limens can be determined accurately. Computational processes are very refined. Experimental errors as found in method of limits are avoided.

DISADVANTAGES The method is not as economical as other methods. Various kinds of biases in judgment can be there.

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