Currently, your knowledge of your main job in the Army bears no relevance on your ablilty to get promoted. I work in a dangerous MOS, around high voltage electricity, and a significant number of personnel are dangerously underqualified. It is literally a matter of life and death, as an incorrect connection will kill someone. But, instead of being promoted on knowledge, or even a hands on exam, they are promoted based soley on their ablilty to shoot, do PT, and complete some form of education which is not required to be in their field.
1SG, I had an occasion several years ago that illustrates your point. I had an E6 who was evaluated and recommended for participation in the E7 advancement exam process (SWE). It was clear to me that he did not meet the standards of performance required to become an E7 on the off chance that he rated high enough against his peer group scheduled for the SWE that fall. I declined to endorse his request for participation in the SWE, and counseled his direct supervisor as to the specifics offering some constructive input as to needed changes. The command backed my decision. Case closed......so I thought.
The following evaluation cycle, his supervisor recommended him for participation in the SWE despite no improvement in his performance, and once again, I declined to endorse the recommendation based on the same solid facts as the previous year. The command overruled me this time saying that we had hurt his career, and by holding our recommendation for a second year, we would significantly damage his career long term. He had apparently served his punishment by not advancing during the previous cycle, and he was now "entitled" to compete in the process. As fate would have it, a weak field overall pushed his final multiple (line score) to near the top and he advanced to E7 where he promptly imploded.
We do our people and the organization a disservice if we don't evaluate people fairly, and hold them accountable. The organization suffers because the best candidates don't necessarily get the positions they should, and members who benefit from a fluffed up evaluation more often than not don't have the skills to perform the job they inherit.
Thanks for letting me rant.
I honestly like the OER system now that I have become more familiar with it here at OCS. I like the fact that if you have 4 Squad Leaders that you rate, you could rank them from 1 to 4 and that would show a board that in a Platoon, said individual was only the 3rd or 4th best Squad Leader. It's still not a perfect system by any means, but much better than the NCOER system in my opinion.
At one time the Navy used a similar system where a ship's CO rated his department heads (senior officer corps onboard) from 1 through 4 or 5 depending on how the ship's ORPLAN was established. The idea was to rate people against their immediate peer group.
The complaint that was that it pitted peers against each other inside the command because there could only be one top dog. It also didn't rate people against people in the same specialty at other commands. The officer writing the article I read describing the system was not in favor of it. His experience was the negative effects from the competition outweighed the benefits, and average performers in poor environments often got better ratings than good performers in very strong commands.
That said, it is clear to most of us who have used any military personnel evaluation system that there are inherent flaws, and that far too frequently, the rater can skew the results any way they want. It's on us as leaders to ensure the integrity of the system....something that doesn't always happen.

NCOER
OER
Army
