(Inspired by a scientific paper quote in this article about autism traits in ADHD, "structural abnormalities in the brains’ white matter nerve bundles were associated with more severe symptoms of both ADHD and ASD.")
First, a brief introduction to experimental design. In biological (and especially biomedical) research, our experimental questions often boil down to "what changes when we do X?" X can be a new drug we're testing, a deletion of a gene, an exercise regime, really just about anything. There's a lot being hidden inside the phrase "what changes," of course: what do you measure; how do you measure it; when do you measure it; when do you decide what, how, and when to measure in the first place. That's not the point of this morning (well, now early afternoon) ramble, though. I want to focus specifically on "changes," themself. Let's walk through a hypothetical situation.