Psychophysical Scaling

1. Psychophysical Scaling Fechner’s Scaling Stevens’ Scaling Psychophysical Scaling

2. Objectives: Be Able to: • Discuss the process of Fechner’s scaling and its weaknesses. • Discuss the process of Stevens’s scaling and its weaknesses. • Compare and contrast Fechner’s and Stevens’s scaling. • Give and explain a practical example of Stevens’s scaling. Psychophysical Scaling

3. Fechner’s Scaling • Psychological Scale of jnd’s because all jnd’s are equal psychological steps. • S = k log R, where S is the psychological sensation as shown by the response, k is a constant determined by the person and the type of stimulus, and R is the stimulus intensity. Psychophysical Scaling

4. Logarithms • Number Log10 Log to the base 10 • 1 0 • 10 1 • 100 2 • 1,000 3 • 10,000 4 Psychophysical Scaling

5. Example of Fechner’s Scaling • Lifted weight in grams Response • 1 .01 • 10 .1 • 100 1 • 1000 10 Psychophysical Scaling

6. Problems with Fechner’s Scaling • Accurate in the middle range of stimulus intensities. • For very low intensity stimuli, Fechner’s scaling underestimates the subjective response, S. • For very high intensity stimuli, Fechner’s scaling overestimates the subjective response, S. Psychophysical Scaling

7. Stevens’ Scaling • Scaling of magnitude estimates, not jnd’s • Magnitude estimation consists of first experiencing a value of stimulus on the scale. The experimenter assigns it a value and you are to accept that value as the magnitude of the stimulus. Subsequent stimuli are assigned a value by you. Psychophysical Scaling

8. Stevens’ Scaling Formula • S = a In where S is the person’s magnitude estimation response, a is a constant dependent upon the person, I is the stimulus intensity, and n is a constant dependent upon the type of stimulus. • For example: for brightness, n = .33 • For loudness, n = .67 • For line length, n = 1 • For electric shock, n = 3.5 Psychophysical Scaling

9. Discussion Question 8 • How are the power law exponents for brightness (.33), line length (1), and electric shock (3.5) related to our experiences with these stimuli? Be sure to include the formula for the power law in your answer. Psychophysical Scaling

10. Scaling Exercise • You will see dots within a square. • I will give you a standard of dots within a square, to which you will assign the value of 10. • Then you will see a series of squares with dots. Some will have more dots than the standard; some, less. • Your task is to make a magnitude estimation of the dots. Psychophysical Scaling

11. Psychophysical Scaling

12. First, write the letters A through H on your paper. • Then we will see the Standard. • Assign it a value of 10. • After that, Stimuli A through H will be presented, one at a time. • Assign each a value. Use decimals for values smaller than 1. Psychophysical Scaling

13. Standard Psychophysical Scaling

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31. Drawing the Graph • Now plot your data on the log-log paper from your course pack. • The stimulus letters are on the bottom and the magnitude estimation values are on the side. • After you plot the data, draw the best-fitting straight line with a ruler. Do not connect the points. Psychophysical Scaling

32. Advantages of Stevens’ Scaling • Magnitude Estimation means that Stevens’ scaling is direct, you plot the person’s responses. Fechner’s scaling is indirect, you must first calculate the jnd’s. Thus, Fechner’s scaling is indirect. • Stevens’ scaling is accurate for very low intensities and very high intensities. Psychophysical Scaling

33. Limitations of Both Types • Must use intensive dimensions, such as brightness and loudness – called prothetic dimensions. • Cannot use qualitative dimensions such as color and pitch – called metathetic dimensions. Psychophysical Scaling

34. Discussion Question 9 • Compare and contrast Stevens’ scaling with Fechner’s scaling. Include the date of Fechner’ s insight and the formula for each scaling procedure in your answer. Psychophysical Scaling

35. Applications • Scale the recommended punishment for robbery as a function of the amount stolen. • Scale sweetness as a function of the amount of sugar added to a dish. • Can you think of any others? Psychophysical Scaling

36. Discussion Question 10 • Give an example of a practical application in which Stevens’ scaling would be useful. Be sure to describe what is measured, give an example of the practical application, and show how the application fits what is measured. Psychophysical Scaling

37. Q 10 • What is measured? • The mathematical relationship between a number of stimulus intensities and psychological responses to them. • An example. • Fit. Psychophysical Scaling

38. Objectives: Be Able to: • Discuss the process of Fechner’s scaling and its weaknesses. • Discuss the process of Stevens’s scaling and its weaknesses. • Compare and contrast Fechner’s and Stevens’s scaling. • Give and explain a practical example of Stevens’s scaling. Psychophysical Scaling