1. Byron Ellis is Senior Fraud Statistician, AdBrite, Inc., San Francisco, CA 94103 . Wing Hung Wong is Professor of Statistics and Professor of Health Research and Policy, Department of Statistics, Stanford University, Stanford, CA 94305 . This research was supported by National Science Foundation grant DMS0505732 (to W.H.W.). The authors thank Rob Tibshirani for pointing them toward multiparameter flow cytometry as a rich source of data and Trevor Hastie for suggesting the cross-validation approach for...