John R. Callahan, Don Hopkins, Mark D. Weiser, Ben Shneiderman. An empirical comparison of pie vs. linear menus. In J. J. O'Hare, editor, Proceedings of the SIGCHI Conference on Human Factors in Computing Systems, CHI 1988, Washington, D.C., USA, May 15-19, 1988. pages 95-100, ACM, 1988. [doi]
Abstract is missing.