The past few years, several researchers have delved into patent statistics to try to determine how likely it is for a patent application to result in an issued patent. Figures have ranged from 62% (USPTO Statistics) to 97% (Quillen). A middle of the road statistic of 75% has also been proffered by Robert Clarke of the USPTO.
In a recent article (Chicago-Kent Journal of Intellectual Property), Lawrence Ebert discusses the various results and concludes that Clarke's 75% estimate is the most reliable because it avoids a double-counting flaw associated with Quillen.
Conclusion: Given no knowledge about the particular invention, a patent application has a 75% chance of resulting in a patent on the invention.