So far in 2012, the USPTO has issued just about 200,000 utility patents. As part of the application process, patent applicants have a duty to identify prior art references that are material to patentability. About 13% of applicants submit no prior art. The median submission is 8 references. However, the data is highly skewed. While most entities submit a relatively small number of references, there are a few entities that submit a large number of references for consideration at the USPTO. Amazingly, the top ten percent of patents (in terms of references cited) are associated with over two-thirds of the seven million prior art citations in this set. Although the median number of references cited by applicants is 8, this skew shifts the average way up to 36 references. The submission of four or five references can be quite helpful. The submission of forty references is unlikely to be helpful (absent a second level search tool). In the past, this has prompted the USPTO to consider mechanisms to place additional burden on applicants who feel the need to submit such a large number of documents for consideration.
Citation also varies by country of origin. The chart below shows average applicant citation count according to assignee home country.