Question.3126 - Discussion Title: Relationship among the Performance Metrics (a) What is the need for the process capability index (Cpk) when we already have the process capability ratio (Cp)? In other words, discuss why Cp is or is not sufficient. (b) Discuss, with reasoning, how one can calculate the yield of a sequence of processes if the yield of each process in the sequence is known.
Answer Below:
Introduction Process capability in Six Sigma measures the percentage or degree of extent to which the production systems of the firm is able to meet the customer’s quality parameters and satisfaction. It provides details of comparing “the output of an in-control process to the specification limits using capability indices” (itl.nist.gov). It measures the responsiveness of the production system to successfully meet customer’s needs and specifications. (itl.nist.gov) Process Capability Ratio vs. Process Capability Index Process capability Ratio (C p ) suffers from a major weakness that the ratio lays emphasis on mean of the product and it wrongly assumes at times that process data always is in line with normal distribution. To get over with this weakness, it is important to calculate process capability index (C pk ) which takes into account the process center of the distribution. C pk takes into account the process distribution center and provides an important view about the consistency of the performance which is checked against the average performance. Therefore, it can be referred that process capability ratio (C p ) is not sufficient Calculation of yield of sequence of processes In the event where the yield of each process is known after calculation of the defects per unit for individual process, the overall yield of sequence of processes can be determined on the basis of first pass yield. First Pass Yield (FPY) refers to yield before any defects have been corrected. This FPY is utilized to ascertain the yield of sequence of processes if the yield of each process in the sequence is known. References Isixsigma.com (n.d.) Process Capability (Cp, Cpk) and Process Performance (Pp, Ppk) – What is the Difference?. Retrieved from http://www.isixsigma.com/tools-templates/capability-indices- process-capability/process-capability-cp-cpk-and-process-performance-pp-ppk-what-difference/ Itl.nist.gov. (n.d.) What is Process Capability?. Retrieved from http://www.itl.nist.gov/div898/handbook/pmc/section1/pmc16.htm MicQuality.com. (n.d.) Six Sigma Primer. Retrieved from http://www.micquality.com/downloads/six-sigma-summary.pdf Pioneerct.com (n.d.). Process Capability Analysis. Retrieved from http://www.pioneerct.com/whitepapers/processcapability/processcapability.htmlMore Articles From Operation Management
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