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Difference between control and specification limit is as follows: Control Limits defined via UCL and LCL Specification Limits defined via USL and LSL Derived from process (calculated from past measurements) – Voice of process Calculated typically as +- 3sigma from mean Received from customer/ market – Voice of customer. It shows the expected performance Factor in calculating process stability Factor in calculating process capability Appears in control charts and understand variations the process is liable to produce Appears in histograms, box plots and probability plot and helps understand acceptable deviation from the performance target Helps understand process variation Helps understand gaps / defects in process performance vs. expected performance Process control limits can be shifted or changed over time Not much influence to change specification limits For sub groups For item or individual measurements Helps identify and reduce producer’s rejects Helps identify and reduce customer’s rejects If a process is called out as in statistical control, then any variation in the process performance would be attributed to common causes only. This process would be consistent over time. If any process performance is noted beyond the control limits, it would be attributed to special causes and predicting performance for such a process would be difficult. Therefore control limits indicates that majority of the variation (+- 3 standard deviations from the process performance mean) in the process is within these limits and only a small portion of the variation is outside which would be attributable to special causes. Whereas the individual process performance measurements that fall out of customer provided specification limits would be said to not meeting the customer’s expectations. While measuring process capability, we generally use Cp, and Cpk when process is called out as in statistical control. We would use Pp and PPk to measure process performance , generally when process is too new to determine if its in statistical control. Interaction between the control and specification limits: a. if control limits fall within the specification limits This is a good scenario as all variations due to common causes will still meet customer expectation. Process therefore has higher capability to meet customer expectations. Also, in future if the control limits change, process capability may not be as severely affected b. if specification limits fall within the control limits: This indicates that there are some measurements or process performances which are not meeting customer expectations even though they may have been caused by common causes or in other words there are larger number of defects. c. if control and specification limits coincide: While all variations caused due to common causes will meet customer expectations, any variations caused by special causes will not meet customer expectations. d. No overlap between control and specification limits: This then indicates that entire process performance or all the measurements are not meeting customer expectations