From time to time, a potential client will ask what percentage of patent applications handled by a particular patent attorney or firm becomes an issued patent.
This is not an unreasonable question in many circumstances, but does not work well for patent applications.
Good patent attorneys obtain patents for their clients when the invention passes the various tests for patentability. The biggest reasons for patent applications to not make it through the patent process are often beyond the control of the patent attorney. The leading reason why the patent application does not emerge as a patent is that the patent examiner finds relevant prior art and makes rejections that cannot be overcome. This is a bigger risk when a client does not authorize prior art searches before starting the patent application process. While the client may have independently developed a great solution to a pressing problem, the same solution may have occurred to someone else. While necessity may be the mother of invention, there is no guaranteed that necessity has only one child.
In some instances, the patent process changes over time after a patent application was filed and before it was examined. This has happened to a large number of business method patent applications that were on file and awaiting examination as the United States Supreme Court decided a number of cases to create new filters to be applied against those applications.