The authors would like to thank Dan Swets and Shaoyun Chen for providing us the programs for MEF and MDF computation. Thanks also go to Yu Zhong, Kal Rayes, Doug Neal, and Valerie Bolster for making themselves available for the experiments. This work was supported in part by NSF grant No. IRI 9410741 and ONR grant No. N00014-95-1-0637.