Statistical Quality Control derived from America in the 1930s as the industrial application of control chart designed by Dr. W.A. Shewhart . This chart was introduced in the Japanese industry after World War II , mostly as a result of roving lecture Dr. W.E. Deming in 1950 .
Although statistical quality control is still an important technique in Japanese quality control system , this system also has some disadvantages :
• In SQC , acceptable quality level ( AQL ) which determine the products that pass but with a minimum acceptable quality , set 0.5 % , or 1.0 % . The rate was not satisfactory from the standpoint of companies that try to achieve very high quality manufacturers , for example : the number of defects by one in a million . For example : the factory Toyota , quality control purposes is reached one hundred percent or units either zero defect numbers . The reason is simple : even Toyota can produce and sell millions of cars , individually , a customer will only buy one . If the car defective , he will think about and told his friends that Toyota is ” junk ” .
• In the Toyota production system , the excessive inventory by is a form of wastage and therefore not allowed . Besides the production on time ( JIT ) or the ability to meet the changing demands with a minimum booking time is also necessary to reduce the dosage to a minimum . If the work of the defects occur at any stage in the process , production flow will be disrupted and the entire line will stop.
Because these two reasons that , Toyota can not rely on the pilot ‘s just statistics alone , and instead was forced to design a low-cost way of doing surveillance on all units ( ” Total examination ” ) to ensure defect-free .
The use of pilot statistically valid as a whole inspection checks because checks are only used when an operation has been fully stabilized through the maintenance of equipment and utensils carefully , and sporadic defects do not occur . In that case, the distribution of product data variation ( 6 x standard deviations ) is relatively small compared to the tolerance that is designed , and could value the center of the data from the central values of specifications designed will also be small . In these circumstances , the pilot inspection plan will ensure the quality of all units in the launcher.
Then , checks on all units or its equivalent has been replaced by the usual statistical model , as well as their own control in the fabrication process has been developed to replace the examination by the independent watchdog . In both cases, the quality control methods have been replaced by more traditional own supervision on all units to further reduce the number of defective units . Definition of the quality control is called ” jidoka ” or ” autonomasi ” .