1. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 e-mail:
2. Systems Integration Division, National Institute of Standards & Technology, Gaithersburg, MD 20988 e-mail:
3. College of Information Studies, University of Maryland, College Park, MD 20742 e-mail:
4. Donald W. Feddersen Professor of Mechanical Engineering, Purdue University, West Lafayette, IN 47907;
5. Professor Department of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907;
6. Professor Department of Educational Studies, College of Education, Purdue University, West Lafayette, IN 47907 e-mail: