Statistical Process Control (SPC)SPC helps to monitor and control a process. Monitoring and controlling the process ensures that it operates at its full potential. At its full potential, the process can make as much conforming product as possible with a minimum wasteProducts conforming to specification are acceptable products
Statistical Process Control (SPC)Two phases of SPCUnderstanding the process variationMonitoring and ControllingFinding the trendToo earlyToo lateKey factor is the cause of variation: Common Cause or Special Cause
Statistical Process Control (SPC)Common Cause or Special CauseHelp us in understanding when and when not to take action.
Statistical Process Control (SPC)Few Causes Each Having Significant ImpactEconomically viable to eliminateMany Causes Each Having minimum ImpactUn-economical to eliminateSPECIAL CAUSESCOMMON CAUSESAlso Called:Random, Chance, Non-assignableAlso Called:Signal, Systematic, Assignable
Selection of VariablesWhat are you interested in?Is this an important characteristic leading value to client?Is client specifically asking to control a particular variable?Is this the most difficult to maintain?
Statistical Process Control (SPC)Univariate Control ChartsIndividual chart for key characteristicsMultivariate Control chartsMonitoring multiple parameters on a single control chartT² Hotelling method is used to generate multivariate charts. Details of multivariate are not part of this course
Rational SubgroupingSubgroupsIn control chart we select some number of units (say 5 units) each time, summarize the variable and plot it on the control chart.For example in X-bar, R chart, we calculate the mean and range of these subgroup variables. Subgroup is the snapshot of the process at that time.Measurements within a subgroup must be taken close together in time but still be independent of each other.
Rational SubgroupingTwo types of variation:Within subgroup:the variation within subgroups is because of common cause variation.Between subgroup:the variation between subgroups is caused by special causes.
Rational SubgroupingControl Limits (UCL, LCL) are calculated based on variation “within” subgroups.Subgroup should be from single stable source. Too much variation in subgroup will lead to too wide control limits.Subgroups should be time based and not randomly selected from a already produced items. This is a time snapshot of the process.
Rational SubgroupingSubgroup size:Subgroup size of 5 is more commonSmall subgroup size > meaningful process shifts may go undetected. Too large subgroup size > insignificant process shifts gives false alarm