K. Ala-Mutka, M. Bacigalupo, S. Kluzer, C. Pascu, Y. Punie, and C. Redecker. Report on a validation and policy options workshop organised by IPTS Seville, 29-30 October 2008, EUR 23786 EN. Office for Official Publications of the European Communities, Luxembourg, (2009)
J. Noonan, and O. Shearer. Cross-Cutting Study, November 1997 - September 1998, U.S. Department of Transportation, U.S. Department of Transportation November 1997 - September 1998 Federal Highways Administration Intelligent Transportation System Joint Program Office 400 Seventh Street, SW, Room 3422 HVH-1 Washington, DC 20590, (September 1998)
M. Maechler, P. Rousseeuw, A. Struyf, and M. Hubert. (2005)Rousseeuw et al provided the S original which has been ported to R by Kurt Hornik and has since been enhanced by Martin Maechler: speed improvements, silhouette() functionality, bug fixes, etc. See the 'Changelog' file (in the package source).