On the computation and finite sample behavior of the constrained M-estimates for multivariate location and scatter

Constrained M (CM) estimates of multivariate location and scatter [Kent, J. T., Tyler, D. E. (1996). Constrained M-estimation for multivariate location and scatter. Ann. Statist. 24:1346-1370] are defined as the global minimum of an objective function subject to a constraint. These estimates combine the good global robustness properties of the S estimates and the good local robustness properties of the redescending M estimates. The CM estimates are not explicitly defined. Numerical methods have to be used to compute the CM estimates. In this paper, we give an algorithm to compute the CM estimates. Using the algorithm, we give a small simulation study to demonstrate the capability of the algorithm finding the CM estimates, and also to explore the finite sample behavior of the CM estimates. We also use the CM estimators to estimate the location and scatter parameters of some multivariate data sets to see the performance of the CM estimates dealing with the real data sets that may contain outliers.National Science Foundation: IRI 95-30546, 06/15/96-05/31/99 Universiteit StellenboschThe author thanks the Editor-in-chief for his comments and suggestions which helped to improve the presentation of this paper. The support of the National Science Foundation under the grant IRI 95-30546, 06/15/96-05/31/99 (P. Meer, J. Cabrera, D. Tyler, Rutgers University, NJ, US) is gratefully acknowledged