"Experience"
Some years ago I was involved in training a number of F/E's for the RHS. Their jobs were evaporating and their company provided conversion courses for those able to take them. They were all very experienced operators though results went from one end of the spectrum to the other. However, one quote remains stuck in my mind - "For years I have sat there thinking 'You stupid old @art, why don't you do it this way?'. Now I know why - and I apologise to them all!"
Prober