The method of adjustment in psychophysics is a technique where the participant directly controls the stimulus intensity and adjusts it until it reaches a specific perceptual criterion, such as the threshold of detection or equality with another stimulus.
In more detail, the method of adjustment, also sometimes called the method of average error, involves the following:
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Participant Control: The key feature is that the participant has direct control over the stimulus intensity. They can increase or decrease the intensity using a knob, slider, or other input device.
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Adjustment to Criterion: The participant's task is to adjust the stimulus until it meets a pre-defined perceptual criterion. Common criteria include:
- Threshold of Detection: Adjusting the stimulus until it is just barely detectable against a background.
- Point of Subjective Equality (PSE): Adjusting the stimulus until it appears to be equal in intensity or some other attribute (e.g., brightness, loudness) to a reference stimulus.
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Multiple Trials: The adjustment process is repeated multiple times. This is crucial for obtaining a reliable estimate of the threshold or PSE. The starting intensity of the stimulus is varied randomly across trials to avoid response biases.
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Averaging for Accuracy: The experimenter records the final stimulus intensity set by the participant on each trial. The results are then averaged across all trials to estimate the threshold or PSE. This averaging process helps to minimize the impact of random errors and individual variability.
Example:
Imagine you're determining someone's hearing threshold for a specific tone frequency. Using the method of adjustment, you would:
- Give the participant a dial that controls the volume of the tone.
- Instruct them to start with the tone at a very low volume (below their expected threshold).
- Ask them to slowly increase the volume until they can just barely hear the tone.
- Record the volume level they selected.
- Repeat this process many times, starting with different initial volume levels.
- Calculate the average of all the volume levels recorded. This average would be an estimate of their hearing threshold for that tone frequency.
Advantages of the Method of Adjustment:
- Intuitive and Engaging: Participants often find the method of adjustment more intuitive and engaging than other psychophysical methods because they have direct control over the stimulus.
- Relatively Fast: It can be a relatively quick way to collect data, especially compared to methods like the method of constant stimuli.
Disadvantages of the Method of Adjustment:
- Subject to Biases: Participants may introduce biases, such as a tendency to overshoot or undershoot the target criterion. Careful instructions and practice trials are needed to minimize these biases.
- Less Precise: Compared to methods like the method of constant stimuli, the method of adjustment can be less precise, particularly if participants are not well-trained or if the task is difficult.
In summary, the method of adjustment provides a straightforward and intuitive way to measure perceptual thresholds or determine points of subjective equality by allowing participants to actively adjust a stimulus to meet a specific criterion.
