Lab Session II:  Forced-Choice Methods and Magnitude Estimation

 

Background:

  1. Purpose and Goals
    1. To illustrate a different way of thinking about human sensitivity: Signal Detection Theory
    2. To expand your computational horizons.
  2. Classical Psychophysical Methods
    1. Method of Limits
    2. Method of Constant Stimuli
    3. Method of Adjustment
    4. Limitations of Classical Methods
      1. Can you think of Any?
      2. consider ways that the data may be not accurate or
      3. ways that the subjects behavior might lead to an incorrect measure of threshold?
  3. Forced Choice Methods
    1. There are several versions.  In this class, we will use forced choice with only two alternatives.
    2. During a single trial, there are two stimuli presented.  In an absolute threshold experiment, one of the stimuli is a stimulus intensity of 0 (or no stimulus) and the other is the stimulus at some intensity.
      1. The two stimuli can be presented at the same time but in different locations or at different times, one after the other.
      2. If presenting the stimuli at the same time, but in different locations, which stimulus is in each location is randomly decided.
      3. If presenting the stimuli at different times, which stimulus is presented first is chosen randomly.
    3. The participant's task is different than in the classic methods. 
      1. If the both stimuli are presented at the same time, the task is to indicate where the target (more intense) stimulus is.
      2. If the stimuli are presented one after the other, the task is to indicate if the target stimulus is presented first or second.
      3. In both cases, if the participant cannot be sure when or where the target stimulus is, the participant must guess.
    4. This task is considered to be a pure measure of a participants sensitivity without any input from a cognitive bias.
    5. Threshold is the stimulus intensity that can be correctly identified 75% of the time.
  4. Magnitude Estimation
    1. All of the methods so far have measured something about perception at or near our limits to either detect a stimulus or a change in the stimulus.
    2. There was a need for a method to try to learn something about stimuli that are easily detectable or the difference between two stimuli that are easily told apart, i.e., is one stimulus twice as bright as another stimulus?
    3. Harvard psychologist, S.S. Stevens pondered this question and basically developed magnitude estimation out of an elevator conversation with another Harvard professor (not a psychologist).
    4. Simple basic idea.  Present a stimulus, have participants give the stimulus a number that they they indicates the sensory strength of the that stimulus.
    5. Modulus:  In some versions, a standard stimulus is used, call the modulus.  This stimulus is given a standard number, whatever the researcher wants, say 50.  Then the participant assigns numbers to the other stimulus that takes the modulus into account.  For example, if the participant thinks the stimulus just presented is twice as strong as the modulus and the modulus is 50 then the subject should give the stimulus a 100.
    6. More in Chapter 2 of the text.

Tasks Due for Next Week:

  1. Do the following experiments:
    1. Forced-Choice Method : Dot Threshold
      1. Leave Stimulus Settings Alone
      2. Adjust the following Method Settings
        1. Type of Method of Constant Stimuli: Forced-Choice
        2. Number of Levels of Relative Dot Luminance: 7
        3. Number of Repetitions: 7
    2. Magnitude Estimation: Tone Loudness & Magnitude Estimation: Line Length from Chapter 2 Media
      1. Number of Levels to Test: 10
      2. Number of Repetitions: 7
      3. Use the modulus (leave checked which is the default).  Note the value.
  2. Problems:
    1. Download Data from all Experiments: Forced Choice and both sets for Magnitude estimation
    2. For Forced-Choice: Show work calculating Threshold with the linear interpolation equation
    3. For Magnitude Estimation: create graph in Excel
      1. with both data sets on one graph as separate lines.
      2. The Physical Dimension goes on the x-axis and the Magnitude Estimate (Psychological) goes on the y-axis.
      3. Give axis titles but in this case you do not need units because the physical dimensions are relative, so the units cancel out, and the Magnitude Estimates do not have units.
      4. Add Legend since two lines.
      5. Edit the graph to have black axis lines (both y and x), remove the grid lines and the graph title.
    4. Enter all information into the Moodle Quiz.
    5. Answer the following question:
      1. Ok, you have uploaded your Magnitude Estimation graph. Now, look at the graph. Really look at it and look at the shape of the two sets of data (line length and loudness).  Do the two data sets look the same.  They have, because I designed it that way, the same x values. But do they have the same shape. Again, the correctness of your answer depends on your data, not some general conclusion. Look at it and see if the two data sets have the same shape or not.  Explain your answer and refer to your graph to support your answer.
    6. Worth 25 points
    7. Do figures in Excel and place in a word file and then type the answers to the questions on the same page.  These may not be hand written.
    8. Point: Learn about making graphs and reading them.

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