Please give me a brief description about statistical process control?

its a concept related to my project in Mechanical Engg. ... (production line)Please give me a brief description about statistical process control?
Classical quality control was achieved by inspecting 100% of the finished product and accepting or rejecting each item based on how well the item met specifications. In contrast, statistical process control uses statistical tools to observe the performance of the production line to predict significant deviations that may result in rejected products.





The underlying assumption is that there is variability in any production process: The process produces products whose properties vary slightly from their designed values, even when the production line is running normally, and these variances can be analyzed statistically to control the process. For example, a breakfast cereal packaging line may be designed to fill each cereal box with 500 grams of product, but some boxes will have slightly more than 500 grams, and some will have slightly less, in accordance with a distribution of net weights. If the production process, its inputs, or its environment changes (for example, the machines doing the manufacture begin to wear) this distribution can change. For example, as its cams and pulleys wear out, the cereal filling machine may start putting more cereal into each box than specified. If this change is allowed to continue unchecked, more and more product will be produced that fall outside the tolerances of the manufacturer or consumer, resulting in waste. While in this case, the waste is in the form of ';free'; product for the consumer, typically waste consists of rework or scrap.





By using statistical tools, the quality engineer responsible for the production line can troubleshoot the root cause of the variation that has crept in to the process and correct the problem.