From what groups do patent citations come from?

There is a communication relating to patent value, based on work by ADAM R. CASTOR , now about to get a Ph.D. at Penn and formerly at the University of Wyoming, with a B.S. Mathematics & Economics, May 2002


The paper titled “Cognitive Neighborhoods and the Valuation of Innovation: A Cross-National Analysis,” asserts that a patent's value can be correlated with the classification code assigned by the USPTO. Yes, patent citations appear:
Castor reported that the difference in terms of what technologies get cited more often than not comes from citations outside the class.

Castor noted:


So I think it’s clear that the classes are important; the next step is to understand exactly how much agency, and what can applicants do in order to ensure that their applications are going to be funneled into the classes that they want, and be seen by the proper examiners.


Taylor Wry noted:


The only thing I would add to that is that, I think one of the unique things about this work that we are doing is instead of looking at individual categories or individual classifications, which is the traditional approach in the academic literature, we are really highlighting the influence of the properties of the classification system.



link: http://knowledge.wharton.upenn.edu/article/location-location-location-adds-patents-valuation/

LBE has commented on the lack of value of patent citation statistics. See for example:

http://ipbiz.blogspot.com/2004/09/there-they-go-again-patent-citation.html

http://ipbiz.blogspot.com/2010/08/citations-found-in-patents-do.html