References
- SCHOEFFLER , M. S. , AND SHERIDAN , J. A. , 1907 , Laboratory studies related to subscriber international dialling. 2 Confusability of audible tones . The Third International Symposium on Human Factors in Telephony, Netherlands Postal and Telecommunications Services .
- WOODSON , W. E. , AND CONOVER , D. W. , 1964 , Human Engineering Guide for Equipment Designers ( Los Angeles UNIVERSITY or CALIFORNIA PRESS ), pp. 1 – 30 , 31 .