Scott Feld talks about research on how people find physicians,

plumbers, mechanics, etc. presumably through referrals from family, friends,

colleagues, and neighbors. There is an article in American Demographics (July 1995 p. 38-44) about the subject. It mentions a study done by Maritz Marketing Research, that has stats on advice seeking on two of the issues you mentioned (physicians and car mechanics).

Rogers, E. M., & Kincaid, D. L. (1981). Communication Networks: Toward
a New Paradigm for Research. New York: Free Press.

Rogers, Everett M., Diffusion of Innovations, The Free Press, 1983, 1995.

Spring/Summer 2001<http://switch.sjsu.edu>

Switch is proud to announce a new Spring/Summer 2001 issue, Social Networks II, v7n1(http://switch.sjsu.edu). Taking inspiration from our previous issue,

Social Networks, v6n2, Switch continues the investigation of what social, networks, and social networks means in relationship to contemporary art practice, new media, technology, and science.

Valdis Krebs valdis@orgnet.com<http://www.orgnet.com>: Our methods of social structure analysis provide insights into ecosystems

and other systems with live actors. Our algorithms ALSO fit well into networks

of inanimate objects such as computers and routers. Guy Hagen has taken social

network concepts and developed 'optimization' algorithms for computer

networks. See: http://www.networkinsight.net/optimize.htm