The trimean is defined to be
See also Hinge, L-Estimate, Mean, Median (Statistics)
References
Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; and Vetterling, W. T.
Numerical Recipes in FORTRAN: The Art of Scientific Computing, 2nd ed. Cambridge, England:
Cambridge University Press, p. 694, 1992.
Tukey, J. W. Explanatory Data Analysis. Reading, MA: Addison-Wesley, pp. 46-47, 1977.