This is one of several slide rules designed for use in psychology. It has a white plastic envelope that holds a paper slide. The upper part of the envelope has scales labeled A (a 23-centimeter ruler, divided to millimeters), N (number of cases), PE (probable error, running from 1 to 10), sigma (running from 1 sigma to 10 sigma), and c (probability that an event happened by chance). The top part of the slide has scales labeled r (running from 0.97 down to 0.00, divided logarithmically) and B (logarithmically divided from 0.05 to 0.20).
The lower part of the envelope has scales labeled sigma (standard deviation) and D (a standard slide rule logarithmic scale for multiplication, running from 10 to 140). The lower part of the slide has scales labeled sigma1.2, sigmaM (standard error of measurement), and C (divided logarithmically and running from 10 to 140). The center of the slide has the line graph r1/2 /III to r1I. The back of the envelope has five more line graphs.
Instructions are provided on the back of the slide and on a leaflet received with the rule. The lower left corner of the envelope is marked: Designed by Jack W. Dunlap and Albert K. Kurtz. The lower right corner is marked: Copyright 1933 by The Psychological Corporation. James McKeen Cattell, Robert Sessions Woodworth, and Edward Lee Thorndike founded the Psychological Corporation in New York City in 1921 to market psychological tests and educational materials. Jack W. Dunlap (1902–1977) and Albert Kenneth Kurtz (1904–1992) were academic psychologists interested in graphic computation charts. They copyrighted the D-K Statistical Slide Rule again in 1938, after which it was sold as a duplex wooden rule. After World War II, Dunlap went into industrial psychology.
References: advertisement, Science 79, no. 2041 (February 9, 1934), back matter, 7; Library of Congress Copyright Office, Catalog of Copyright Entries: Part 1, Books, Group 2, Pamphlets, Etc., n.s. 31 (1935): 24; Jack W. Dunlap and Albert K. Kurtz, Handbook of Statistical Nomographs, Tables, and Formulas (Yonkers-on-Hudson, N.Y.: World Book Company, 1932); Albert K. Kurtz, "Obituary: Jack W. Dunlap," American Psychologist 34, no. 6 (June 1979): 538; J. McKeen Cattell, "The Psychological Corporation," Annals of the American Academy of Political and Social Science 110 (November 1923): 165–171.
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