Precisely! You need a clearly defined state of affairs to use as a baseline.
True, but I don't think there's any reason to not "brainstorm" about perceived changes/impact before we get "all scientific" about things, because there could well be patterns we miss if we don't evaluate the perceived-changes/perceived-impact on rankings.
If we only allow "carefully tracked" changes and what seem to be "snap reactions" by Google's algo, we could miss: Webcentric made URL changes that seemed to have a negative reaction algorithmically, while I made changes to URLs, titles, H1s, page content, etc. that seemed to cause a temporary boost in rankings.
There was another fairly recent thread about someone adding H1s to pages and dropping in the rankings, but I changed H1s and got a boost, so imo, it's possible there are "thresholds" for changes -- Maybe, possibly, if you only change one thing, algorithmically it looks like a test/manipulation but if the template, source-code, titles, H1, content, etc. are all changed at the same time it looks like an upgrade algorithmically?
Bottom line for me is: I think we need more feedback on what seems [appears]
to have impacted rankings before we "get scientific" about things, because like netmeg said, there are infinite possibilities, and more feedback/examples of possible cause/effect situations give us a better idea of what to test.