On the Loss of Reliability in Ratings Due to Coarseness of the Scale.

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Abstract
After a discussion of the various ways in which scales may be constructed, the writer concludes that "for rating traits of personality the optimum number of class intervals is 7. Rating with a scale of more than this number of classes demands a discrimination which does not yield an increase in reliability sufficiently great to make the increase worth while according to an arbitrary definition of such worthwhileness. Likewise, according to our definition, a rating scale with a fewer number of classes suffers from a loss of reliability greater than is allowed, due to coarseness of grouping. The graphic rating scale, although permitting as close a discrimination as possible, contributes but little to increased reliability over a 7-point scale. Group mental and scholastic tests which cover a range of score of a hundred or more points give an illusory impression of discrimination." From Psych Bulletin 22:12:00912. (PsycINFO Database Record (c) 2006 APA, all rights reserved)

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