Let's use a little logic: 2+2=4, 3+3=5. 100% of us can agree that one answer is correct and the other is not. This mutual agreement is because this statement is quantifiable and it is definitive not subjective.
Dressage is "subjective" not "definitive". The USDF has done a great job trying to standardize the meaning of correct training and a correct horse. They have made it more definitive by quantifying it through tests, but it is still subjective.
The word "correct" is "not" subjective, it is "definitive". The word "correct" is undeniable not arguable. It is black or white. If you video tape a horse in a test and send it to 5 different USDF judges, you will receive 5 different scores because it is open to argument. A correct horse would receive 100% of 100% judges that judged that particular horse 100% of the time. To my knowledge, this has never happened and probably will never happen, because there is no such thing as a correct horse. If there were a correct horse, we would have an example of how to train a correct horse.
So Western Dressage is "not" how "correct" your horse is trained or the movement of, it is the method of how well the "quality" of the training is. You are judged on the degree of "quality" , not "correctness" and judging is a validation of how well you are communicating with a non human being.
Here is an example of measurement of degree of "quality" not "correctness". It is probable 100% of the dressage world would agree that this horse and rider are not correct, because they are only 68.3% correct. Face it, the rider has terrible equitation and the horse and rider make mistakes, who cares because it's not about "ego", its about the "partnership" of the team. Right or wrong, it is what it is.
On April 14th, 2012 Randy Byers performed a Western Dressage test at Donida Farms in Auburn Washington. He received a 68.3 on USEF Primary Western Dressage test 4