# Method of Constant Stimuli

Estimate of the Point of Subjective Equality from the obtained graph: PSE = 101.6 Estimate of the upper threshold (UT) from the obtained graph: UT = 104.2 Estimate of the lower threshold (LT) from the obtained graph: LT = 98.5

Estimate of the interval of uncertainty (IU) from the obtained graph: IU = upper threshold – lower threshold, which in this case is: 104.2 – 98.5 = 5.7. Therefore the IU = 5.7

Question 5

Estimate of the just noticeable difference from the graph obtained:

Question 6

Answer B- it would be smaller

Question 7

Calculation of UT – PSE from the obtained data

Upper threshold – Point of Subjective Equality

104.2 – 101.6 = 2.6

Question 8

Calculation of PSE – LT from the obtained data

Point of Subjective Equality – Lower Threshold

101.6 – 98.5 = 3.1

## Question 9

Mean average of the 2 estimates above (UT – PSE and PSE – LT) does equal to the computed JND in question 5

Mean of the 2 estimates = (the calculated JND in question 5)

## Method of Adjustment

## Question 1

## Trial No.

## Length (X)

## (Length – Mean)

## (Length – Mean)²

1

98

(98-100.75) = -2.75

7.6

2

101

(101- 100.75) = 0.25

0.1

3

97

(97-100.75) = -3.75

14.1

4

103

(103- 100.75) = 2.25

5.1

5

99

(99-100.75) = -1.75

3.1

6

101

(101-100.75) = 0.25

0.1

7

100

(100-100.75) = -0.75

0.6

8

102

(102-100.75) = 1.25

1.6

9

97

(97-100.75) = -3.75

14.1

10

105

(105-100.75) = 4.25

18.1

11

99

(99-100.75) = -1.75

3.1

12

102

(102-100.75) = 1.25

1.6

13

102

(102-100.75) = 1.25

1.6

14

103

(103- 100.75) = 2.25

5.1

15

103

(103- 100.75) = 2.25

5.1

16

99

(99-100.75) = -1.75

3.1

17

103

(103- 100.75) = 2.25

5.1

18

103

(103- 100.75) = 2.25

5.1

19

97

(97-100.75) = -3.75

14.1

20

101

(101-100.75) = 0.25

0.1

Sum of length = 2015

Sum of (length – mean)² = 108.5

Mean = 100.75

Mean =

## Question 2

To calculate the standard deviation, the formula for variance can be used. However to obtain the standard deviation, variance must be square rooted.

Variance = SD =

So standard deviation in this case is equal to:

SD =

## Question 3

In the Method of Adjustment, the just noticeable difference (JND) is calculated by multiplying the standard deviation (SD) by 0.6745

so JND = (SD) × 0.6745

= (2.3) × 0.6725

= 1.55

## Question 4

In the Method of Adjustment the point of subjective equality (PSE) is equal to the mean of the adjusted lines; in this case 100.75

## Question 5

In the Method of Adjustment the upper threshold is calculated by adding one JND to the PSE.

Upper Threshold = Point of Subjective Equality + Just Noticeable Difference

= 100.75 + 1.55

= 102.3

## Question 6

In the Method of Adjustment the lower threshold is calculated by subtracting one JND from the PSE.

Upper Threshold = Point of Subjective Equality - Just Noticeable Difference

= 100.75 - 1.55

= 99.2

## Question 7

In the Method of Adjustment the interval of uncertainty (IU) is the difference between the upper threshold and the lower threshold.

IU = UT – LT

= 102.3 – 99.3

= 3

## Question 8

The correct answer is D.

## Question 9

The correct answer is B.

## Question 10

The correct answer is D.

## Weber’s Law Worksheet

## Question 1

## Standard Number

## Standard Size

## PSE

## JND

1

40

39.49

0.56

2

120

121.37

2.75

3

200

204.00

3.88

To calculate the slope, we can subtract the corresponding Y-coordinates of two points on the line, and then divide by the difference between the X coordinated of the same two points. The following formula can be used:

Slope =

## Question 2

To calculate Weber’s fraction we can use a re-arranged equation of Weber’s Law to find the Weber fraction for each stimulus.

For a standard stimulus of 40:

For a standard stimulus of 120:

For a standard stimulus of 200:

Compute an average of all 3 standard stimuli

## Question 3

Yes. The JND value for larger stimuli was larger, and therefore the variable error can also be said to be larger.

## Question 4

No. As the variable error was large for larger stimuli, we can say that we were not more precise when judging larger stimuli. A large variable error represents low precision, while a small variable error predicts high precision.

## Question 5

No.

## Question 6

The correct answer is A.

Using

To solve for or 1kg

## Question 7

The correct answer is C.

Find the K value using

Then solve for JND using

= 0.04 x 400g

= 16g

## Question 8

No. For the Method of Constant Stimuli, 3 different sets of stimuli (lines) need 3 different sets of intervals, as JND for the longer line would be far greater than the JND for the shorter lines.

## Muller-Lyer Illusion Worksheet

Condition Number

Standard Arrow Direction

JND

PSE

Constant Error

1

## >----------<

3.1

109.14

9.14

2

І----------І

2.76

102.29

2.29

3

## <---------->

1.9

98.36

-1.64

## Question 1

Yes. As seen in the table above; the PSE (point of subjective equality) decreases as the standard arrow direction changes.

## Question 2

Constant error is calculated by subtracting the PSE from the POE (point of objective equality); which in this case is 100.

Constant error for condition one = PSE – POE

= 109.14 – 100

= 9.14

## Question 3

Constant error is calculated by subtracting the PSE from the POE (point of objective equality); which in this case is 100.

Constant error for condition two = PSE – POE

= 102.29 – 100

= 2.29

## Question 4

Constant error is calculated by subtracting the PSE from the POE (point of objective equality); which in this case is 100.

Constant error for condition one = PSE – POE

= 98.36 – 100

= -1.64

## Question 5

Condition 3

## Question 6

Condition 3

## Question 7

The conditions are equally accurate.

## Question 8

The correct answer is C.

## Experiment: M-L Illusion with two different lengths of lines

Condition Number

JND

PSE

Constant Error

Arrow Direction

Short

Long

Short

Long

Short

1

## >----------<

1.9

4.21

40.67

151.01

10.67

2

І----------І

1.46

3.92

34.67

140.98

4.67

3

## <---------->

0.8

4.8

30.73

137.59

0.73

Short

Long

POE:

30

140

Number of Trials:

48

48

Top Line:

Both

Both

Horizontal Separation:

Random

Random

Vertical Separation :

240

240

Presentation Order:

Counterbalanced

Counterbalanced

## Question 1

The illusion seemed to have occurred for both conditions. The constant errors for the arrow tails condition (Condition 1) indicate that mean judgments were larger than the real value (the POE). It is the nature of the illusion that the line with the arrow tails is longer. In this case, we can say that the illusion did occur for both conditions as judgments for Condition 1 (short) and Condition 2 (long) were 10.67 and 11.01 respectively. A positive constant error value indicates judgment to be larger than the real value and a negative value represents the mean value to be smaller. Accuracy in both of these conditions was low. Constant error is a measurement of accuracy; and as the Muller- Lyer Illusion states, the illusion itself should produce judgments that are low in accuracy.

## Question 2

Yes. It is known in general, that perceptual constancies, depth cues and several principles of visual organization help humans perceive the world accurately. But sometimes such perceptions are based on unproved assumptions, and what we have come to know as visual illusions can result. The illusion should have occurred regardless of size of the stimulus. Evidence suggests that the accuracy of the decisions we make in regards to the two lines is based on a lifetime of experience with edges and corners of buildings and rooms around us. The horizontal line with the Vs is seen as a corner of a room viewed from the inside; and the line with arrowheads is perceived as a corner of a room or building seen from the outside. It could be that humans use depth cues of 3D space to perceive even a 2D design and therefore the size of stimulus; whether long or short; would not matter.

## Question 3

Human ability to judge differences between stimuli is dependent on the strength of the stimuli to which the person may be exposed to. The weaker the stimuli, the easier it is for humans to detect a small difference between them. This small difference that we can detect is often referred to as the Just Noticeable Difference (JND). Weber’s Law states that the JND (just noticeable difference) is a constant proportion of stimulus intensity. This proportion or fraction in known as K. While K is different for each sense, the smaller the value of K is, the more sensitive a sense is to the stimulus differences. In the case of the Muller- Lyer Illusion, we can calculate the K value for each of the instances used in this experiment for the:

## Short Condition:

## Long Condition:

It seems that according to Weber Law, we can say that indeed the Condition 1 line under the short length condition is harder to adjust, as K is larger and therefore less sensitive to stimulus differences. While the same could be stated for the long length line condition, as the larger K value is corresponding to the 3rd condition line. Therefore, the data obtained does indeed follow Weber’s Law which states that the JND (just noticeable difference) is a constant proportion of stimulus intensity.

## Question 4

Variables that were controlled for in this experiment include: the random presentation of the different lines; counterbalancing the ascending and descending starting positions of the trials and randomizing the horizontal separation between the two lines. Vertical separation and the number of trials were also kept the same under both conditions.

A repeated-measures design refers to studies in which the same measures are gathered on multiple occasions for each subject but under different conditions. While most of the given variables were controlled in this experiment; no measures were taken to control several threats of internal validity of the repeated measures design. Subjects in this experiment were asked to generate their data as closely as possible to the ideal data; meaning that subjects were tested on several occasions and therefore more than once. Scores therefore may have tended to cluster towards the mean and allow participants to obtain more ideal data. Practice effects also tend to surface and influence results when participants in a repeated measures experiment perform the given task well and then are asked to perform it again at a different time. It is very likely that performing the task previously would have influenced later results, either having a positive or negative effect on results.

Results of the experiment could have also been affected by a history threat; simply meaning that events outside of the experiment may have influences the responses of the participant. Additionally, no exact standard condition was used for this experiment. Any psychological research experiment using a repeated-measures design should assign each patient randomly to a sequence of treatments. Normally, a standard treatment or a placebo is used in a repeated measures experiment. Participants cross over from one treatment to another; providing a balance, meaning that each participant is exposed at the same number of treatments and that all subjects participate for the same amount of time.

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