And speaking of correlation, that is a big bugaboo. Correlating test equipment is the biggest pain ever. The question is; how can two piece of test equipment, both being calibrated give two different measurements for the same part? The answer is yes. That's it. Yes. So the old saying goes; "a man with two watches is never sure what time it is." A lot of time and effort spent over the years getting test equipment in two or more locations to correlate.
Then we talk about subjective evaluations and the results will be all over the map. Subjective evaluation is often used where measurements cannot give a complete picture. We do it all the time in the audio world and I'm sure that most if not all audio equipment manufacturers do subjective testing as well. A jury must be trained to listen for specific characteristics and typically receive very specific guidelines detailing how to rate a component or system. In my experience, the environment is closely controlled and the subjective evaluations are repeated by the panel several times over a few days. Our moods and previous environmental exposure can affect the results. So we Audiophiles are typically self taught in the art of listening. These forums and articles can provide instruction as well. I'm not saying I'm a great expert at subjective listening. I'm in my mid 60s now and recently I was at the Tampa Audio Expo with my son who is 31. He could hear things in the various rooms that I just could not pick up. And based on that I am telling myself now that I really need not spend so much on upgrades these days. I have hit my limit of hearing. But I have no doubt that others can hear differences that make sense to upgrade. I have a 45 year history of that.