Another simple method/idea along these same equivalent is to use a "control", reference, or as Vaughn mentions, a baseline.

You need some point of reference to compare your results against.
Aquariums are not easy systems for many hobbyist to make similar to each other.

I use non limiting approaches to rule out things like low PO4 or CO2 etc.
Then I know that growth is not limited due to these other factors.

Only then can I limit and manipulate the variable of interest and get a good handle on what is happening with much confidence.

Folks sometimes wonder how I know, or exudes all this confidence, and am so aggressive in my argument when I read what I know to be a myth.

Heck it's not personal, it's just I've done this and bothered to think about it, test it, made the mistakes and came to a very different conclusion based on the evidence I found.

Not what I want it to be
Some seem to have a fore drawn conclusion based on some idea they read.
Or are not honest about their steps.

Vaughn is being honest and humble.
That is a key for any good experimenter.
More than anything I'd say. You realize how little you do know and are not going to try and make more out of the test than is there.

Yes, many folks get excited and think they found the golden fleece............then only later do they realize that it was not the case and it was something else folks had mentioned all along.

Things like CO2 will get folks even 10 years into the hobby.

One thing that I've been considered is using a different type of test method, more based on observation on systems I cannot manipulate and control. We have little choice in many ecological situations, and we need to draw inferences from lakes, rivers etc. Many aquariums also fall into this group.

But suggesting cause and effect in such systems is very hard at best, there are correlations and models to try and requires a lot of statistical keeness. And I've yet to see any aquarist use this approach.

You also learn how to be a good grower in many tanks, situations over time.
Once you get there, then you can test things much better.

You have more control.

Regards,
Tom Barr