Survivor bias

I guess you all know how this works, right? Airplanes that returned safely from bombing runs were examined. All areas that had suffered bullet holes were considered "non-essential", as the plane still made it back. But areas that had NOT been hit were considered "essential", as they had allowed the airplane to return. That's actually an good way of looking at reality, though not necessarily exhaustive. Ford did the same - they examined old cars and checked which parts had NOT failed - then made those parts weaker.