The next question, that you may ask is – why is then the control limit required ? The control limit indicates how is the process performing over a period of time. deviation can change it is not expected for the control limits to remain the same. Since for every sample data, the mean and the std. If the control limits remains constant, it only means that your measurment system is not good enough to measure the variation. The UCL and LCL are calculated by using the formula : X-bar +/- 3Std. note that the control limit is ‘expected to vary’ and not remain constant. As your process parameter changes over a period of time, the same will get reflected in the control limits. The Control limits (UCL & LCL) that are shown in any control chart, is the voice of the process. This will always remain constant, till any change is made in the SLA(Service Level Agreement) or the control plan or contract. It indicates within what range the customer expects the product parameter to vary. The Specification limits (USL & LSL) is the voice of the customer. First of all, let us understand the Basics of statistics.Ther are basically 2 types of imits we refer to in statistics – the specificatio limits and the control limits.
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