Prometric is the test sponsor for the IRS Special Enrollment Examination.  They periodically publish results (pass rates) for the exam.  The latest pass results can be viewed by clicking on this link.

As you can see by viewing the results, the pass rate for Part 1 is between 50% and 60%; Part 2 is between 50 and 60% with the lastest results for 2016-17 testing period coming in strong at vert close to 60%; Part 3 is between 80% and 90%.

We would expect to see Part 1 (individual focused) to be higher than Part 2 (business focused), but they are not.  This might be explained by Part 1 is taken first by many test takers (though it does not need to be), and when they have a tough experience they do not continue.  Therefore, the pool who takes Part 2 has, in many cases, already shown the ability to pass Part 1.  This is just a theory.