Human resources are the greatest source of variability in nearly ANY business process.
You can provide tools, training, materials and controlled working environment, with all the variability squeezed out of them as is possible, but in walks John Doe Employee, copping an attitude because his wife ruined breakfast and there goes his productivity. My objective, in doing business process design and improvement, is to find or invent process steps that make it easy to do it right and hard to do it wrong.