"There is no statistically significant relationship between project management knowledge as demonstrated by total scores on the knowledge test based on the PMBOK Guide and the measure of perceived effectivemess of workplace performance", at least that is what Lynn Crawford claims are the results of a study published in the International Journal of Project Management (Vol 23, Issue 1, Jan 2005).
The conclusion is that "there is no direct relationship between how well project managers perform against standards for knowledge and use of practices, and how well they are perceived to perform by their supervisors".
It is a bit disappointing that the study relies on supervisor assessment and uses the measure of "perceived effectiveness" rather than some more objective measure. But it is an interesting study and points out some roadblocks which Project Managers need to overcome. When professional associations say one thing is important and the people who pay your salary say another, it is a difficult situation.
