This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

Summary: Psychophysics and Signal Detection

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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