Book T of C
Chap T of C
This is the 2007 version. Click here for the 2017 chapter 04 table of contents.
The history of psychophysics from the early 1800s to the computer age showed changes in the way basic concepts like "threshold" were conceived. Initially, a sensory threshold was assumed to be an all-or-none point at which a signal became detectable. After World War II, scientists realized that an observer picking out a signal from background noise was essentially guessing or making a probabilistic judgment with more or less confidence, depending on how strong was the signal compared to background noise. A person could be led to change the threshold for saying Yes or No by manipulating the experimental situation.
Scientists further realized that if they compared performance of a subject in a variety of biasing conditions, the effects of bias could be separated statistically from the effects of the observer's sensitivity. This resulted in a pure measure of observer sensitivity called d' (d prime). D prime is important because it provides a way of measuring whether a treatment like hypnosis or acupuncture makes an actual change in a subject's sensitivity to stimuli. Before d prime was discovered, scientists could not tell whether a subject was becoming more sensitive to stimuli or simply more willing to report detection of an event. D prime separates these two factors.
The D prime statistic can be used to guide any diagnostic decision involving a Yes/No judgment, such as whether a given piece of luggage might contain a bomb. It is an example of how scientific psychology and the analysis of human decision making can make a difference in the "real world."
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Copyright © 2007-2011 Russ Dewey