控制品质的其中一个环节就是控制公差，这是为什么说将质量嵌入到产品设计当中的原因。公差将直接影响产品的一致性，稳定性，可靠性和功能。公差设计出来比较容易，但是如何将公差控制在一个有限的范围内却是一件困难的事。
One of the steps in controlling quality is to control tolerance. This is why quality should be built in product design. Tolerance will directly influence the consistency, stability, reliability and functions of products. It is easy to design tolerance, yet it is difficult to control the tolerance in a limited range. This is why America cultivated over ten thousands of engineers studying the control in statistical processes to ensure the quality of military products during the 1940s. The aim to control statistical process is to minimize the product defects which exceed the tolerance, and to inform the engineers of the investigation reasons to take precautions via the form of graphic trend in advance before producing defective goods.