An important way to think about this is what actually happens when you do an experiment: i.e. the measurement is ultimately happening at your eye-ball, and is a

An important way to think about this is what actually happens when you do an experiment: i.e. the measurement is ultimately happening at your eye-ball, and is a chain of linked effects we believe we understand all the way down to the item under test (which is part of the reason metrology itself is fascinating).