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The production manager has no control over the price that the company's sole supplier charges for the raw material. The company cannot change suppliers, because there is no other supplier to change to; and the production manager would not have any control over the supplier used even if the company could change suppliers. Therefore, the production manager should not be held responsible for the price variance.
When circumstances change, standards should be adjusted to reflect the changed situation. In this case, there is nothing the company can do about the price increase because it has only one available supplier. Therefore, the company has no choice but to pay the higher price and it should adjust its standards accordingly.
The purchasing manager has no control over the price that the company's sole supplier charges for the raw material. The company cannot change suppliers, because there is no other supplier to change to. Therefore, the purchasing manager should not be held responsible for the price variance.
The material usage standard should not be adjusted to compensate for changes in raw material costs. To do so would compromise the quality of the product.