Operators work with those up the line in order to provide the best possible parts. If our lot sizes are small, when we do discover a problem there are fewer bad parts to deal with. Smaller lot size will also quickly reveal problems in the manufacturing system. The managers at a factory that truly practices JIT will see this as an opportunity for improvement, rather than a problem to avoid. A frequently used analogy is that of ships in a harbor.
Inventory is like the water in the harbor.
If it is high, no one notices the rocks in the harbor bed. If the water level goes down, the rocks become exposed. In the JIT philosophy, one removes the rocks rather than increasing the water level. JIT production systems are pull systems. An MRP push system generates work orders scheduled in anticipation of the need for finished goods.