[21] W.F. Book: The Psychology of Skill. (University of Montana, Publications in Psychology, 1910.)
[22] H. Münsterberg: Beiträge zur experimentellen Psychologie. (Book iv, 1892.)
[23] A.J. Culler: Interference and Adaptability. (Archives of Psychology, 1912.)
[24] The experiments are being conducted and will be published by Mr. L.W. Kline.
[25] Adolf Gerson: Die physiologischen Grundlagen der Arbeitsteilung. (Zeitschrift für Sozialwissenschaft, 1907.)
[26] Karl Bücher: Arbeit und Rhythmus. (Fourth Edition, Leipzig, 1909, p. 438.)
[27] Frank G. Gilbreth: Motion Study. (New York, 1911.)
[28] Taylor: The Principles of Scientific Management. (New York, 1911, p. 71.)
[29] The experiments are being conducted and will be published by Mr. E.R. Riesen.
[30] R. Herbertz: Zur Psychologie des Maschinenschreibens. (Zeitschrift für angewandte Psychologie, 1908, vol. 2, p. 551.)