[15] If desired this form may be used, with the appropriate difference in the form of the questions and statements.
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[16] Courtis finds in the case of addition that "of all the individuals making mistakes at any given time in a class, at least one third, and usually two thirds, will be making mistakes in carrying or copying."
[17] Facts concerning the conditions of learning in general will be found in the author's Educational Psychology, Vol. 2, Chapter 8, or in the Educational Psychology, Briefer Course, Chapter 15.
[18] See Thorndike ['00], King ['07], and Heck ['13].
[19] A special type could be constructed that would use a large type body, say 14 point, with integers in 10 or 12 point and fractions much larger than now.
[20] It will be still better if the 4 is replaced by an open-top 4.
[21] For an account in English of their main findings see Howell ['14], pp. 149-251.
[22] In his How We Think.
[23] Compiled from data on p. 89 of Kruse ['18].