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The semantic differential is a measure of the connotative "distance" between two concepts. For instance, "sun" and "beach" may be closer connotatively than "cornbread" and "blasting caps". It is an important tool for psychologists because it allows them to measure on a consistent scale people's reactions to concepts and ideas.

The Genesis of the Semantic Differential
The technique was developed in the 1950s by a psychologist named Charles Osgood and described in his 1957 book The Measurement of Meaning. Osgood was interested in measuring connotation, and needed a method for doing so. What he did was generate a list of words and ask people to free-associate with them-- to give him the first word that occurred to them upon reading each word in his list. He then took the words from the free-association, determined their opposites, and created a bipolar scale based on them. Now any word could be rated on a series of these bipolar scales: is "dog" closer to "hot" or "cold"; closer to "fast" or "slow"; closer to "happy" or "sad"? Calculating the differences between results for two different words on the individual antonym-based scales provides you with their semantic differential.

The EPA Structure
Another experiment was conducted in which antonym pairs were pulled from Roget's Thesaurus and students were asked to rate a set of concepts along a scale between the two words in each antonym pair. When the correlations were calculated, a structure emerged which also described the results of earlier studies: All variations could be reduced to three basic dimensions, described as Evaluation, Potency, and Activity. "Evaluation" refers to the idea that something is considered good or bad to some degree. "Potency" is a measure of how strong or powerful the subject is considered to be. "Activity" describes the degree to which the subject is considered, well, active. Together, these three facets form what is known as the EPA Structure, which has been shown through subsequent experimentation to hold up in many very distinct cultures.

The Semantic Space
Plotting concepts as points on a graph-- using the three facets of the EPA structure as axes-- one can construct a "semantic space" which visualizes the connotative responses engendered by those concepts. Interesting comparisons and relationships can be drawn among concepts plotted in the semantic space using normal three-dimensional geometry, making it a very useful analytical tool for psychologists.


Sources
Dr. Jack Feldman's PSY2210 class at Georgia Tech
http://www.indiana.edu/~socpsy/papers/AttMeasure/attitude..htm
http://www.writing.ws/reference/history.htm

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