1. Department of Engineering and Public Policy, Carnegie Mellon University , 5000 Forbes Avenue, Baker Hall, Pittsburgh, PA 15213-3815, USA. e-mail: ccombema@andrew.cmu.edu
2. Department of Mechanical Engineering, Carnegie Mellon University , 5000 Forbes Avenue, Baker Hall, Pittsburgh, PA 15213-3815, USA. e-mail: kwhitefoot@cmu.edu
3. Department of Mechanical Engineering, Tepper School of Business, Carnegie Mellon University, Pittsburgh, PA 15213-3815, USA. e-mail: ales@cmu.edu
4. Department of Engineering and Public Policy, Carnegie Mellon University , 5000 Forbes Avenue, Baker Hall, Pittsburgh, PA 15213-3815, USA. e-mail:erhf@andrew.cmu.edu