1. R. A. Fisher. “Statistical Methods for Research Workers”, 4th edition, 1932, p.120 (Oliver & Boyd, London).
2. R. W. Burley. Ph.D. Thesis, Leeds University, 1952.
3. H. Leaderman. “Elastic and Creep Properties of Filamentous Materials and Other High Polymers”, 1943, Washington D.C. (The Textile Foundation).