A new working paper by Juan Alcacer and Michelle Gittelman (Both at NYU) examines prior art citations in patents. One interesting finding:
On a per patent basis, 2/3 of all cited art was discovered by Examiners rather than submitted by Inventors.
Alcacer, Juan and Gittelman, Michelle, “How do I Know what you Know? Patent Examiners and the Generation of Patent Citations“