If you introduce a new training element/stimulus to an athlete’s program and his progress is linear in nature, then all that happens is that his pre-existing bio-motor qualities transfer, and he gets better only because of the technical improvement and the familiarization with it.
I think this has many implication in general coaching
Without some sort of control, you have a problem with measurement. If improvement is linear, you might infer that the new stimulus addition/substitution had no significant effect, but you don’t know if simply changing the stimulus might have avoided a plateau.
If you maintain logs from prior years, you can use the prior log as a control, if you only change one thing in one individual/group. With Charlie’s plan (what’s on the various downloads) you might use the initial plan as a baseline first, then begin to experiment. I’ve done this with incorporating some of the JS/HSI work: Some things worked better (more special endurance in phase II) and some things didn’t (too much of a good thing–too much SEI and not enough speed), but as long as you test just a single hypothesis at a time, you eventually figure out what works and what doesn’t.