Quality patents depend upon our ability to find and examine prior art regardless of its form. Most prior art cited by the USPTO arrives in the form of patent-related references — most notably, issued U.S. patents, published patent applications, and foreign patent documents. We know, for instance, from Columbia Professor Bhaven Sampat that about 90% of cited non-patent prior-art references are provided by the applicant, and that only 10% are examiner references. Link.

In some technologies where patenting has a long and extensive tradition, the lack of non-patent prior art is not a serious problem.  In other areas such as software and biotechnology, non-patent references harbor most of the important prior art. 

Results: My new study of 100,000 recently issued patents reveals that patent that cited non-patent literature are significantly more likely to receive rejections from the PTO.  Specifically, patents citing at least one non-patent reference have a 39% greater chance of receiving a final rejection when compared with patents that issued without citing any non-patent references. Regarding non-final rejections, citing non-patent prior art increased the likelihood of receiving a rejection by 9%.