# 8.pdf - Statistical Process Control(SPC SPC helps to...

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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 waste Products conforming to specification are acceptable products
Statistical Process Control (SPC) Two phases of SPC Understanding the process variation Monitoring and Controlling Finding the trend Too early Too late Key factor is the cause of variation: Common Cause or Special Cause
Statistical Process Control (SPC) Common Cause or Special Cause Help us in understanding when and when not to take action.
Statistical Process Control (SPC) Few Causes Each Having Significant Impact Economically viable to eliminate Many Causes Each Having minimum Impact Un-economical to eliminate SPECIAL CAUSES COMMON CAUSES Also Called: Random, Chance, Non-assignable Also Called: Signal, Systematic, Assignable
Selection of Variables What 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 Charts Individual chart for key characteristics Multivariate Control charts Monitoring multiple parameters on a single control chart T² Hotelling method is used to generate multivariate charts. Details of multivariate are not part of this course
Rational Subgrouping Subgroups In 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 Subgrouping Two 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 Subgrouping Control 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 Subgrouping Subgroup size: Subgroup size of 5 is more common Small subgroup size > meaningful process shifts may go undetected. Too large subgroup size > insignificant process shifts gives false alarm