*A thesis submitted as partial fulfillment of the requirements for the degree of Master of Science in Dentistry in the Department of Orthodontics, School of Dentistry, University of Washington, 1952.
*Grateful acknowledgment is made to the author for this paper written at the request of the Editor. It is hoped that it will serve as a reference article for clarification of the statistical terms used in many papers.
*This investigation was supported (in part) by Research Grant D-658 from the National Institute for Dental Research, National Institute of Health, U.S.P.H.S.