how do you isolate a variable in a complex system

The core of good diagnosis. Your method: 1) You only change ONE thing at a time, so you can attribute the result to that change — change two things and you don't know which one mattered. 2) You keep a baseline so you can compare. 3) You control what you can (conditions, inputs) and observe what you can't. 4) You use the data to confirm or kill hypotheses. The key line: 'If I change two variables and the problem changes, I've learned nothing — so I change one, measure, repeat.' A real example strengthens it: at Onto you ran controlled experiments to isolate the root cause of a systemic issue. This disciplined one-variable-at-a-time approach is exactly the GPS field method. It's a differentiator — most people shotgun-fix.