What is used to periodically check that a process is in statistical control?
Statistical Process Control (SPC) is a statistical method to measure, monitor, and control a process. It is a scientific visual method to monitor, control, and improve the process by eliminating special cause variation in a process. Show
History of Statistical Process Control (SPC)Though SPC effectively used in western industries since 1980, it was started during twenties in America. Walter A Shewhart, developed the control chart and the concept that a process could be in statistical control in 1924 at Bell Telephone Laboratories, USA. The SPC concepts are included in the management philosophy by Dr. W.E. Deming just before World War II. However, SPC became famous after Japanese industries implement the concepts and compete with western industries. Meaning of SPC
Why use Statistical Process ControlToday companies are facing increasing competition and also operational costs, including raw material continuously increasing. So for the organizations, it is beneficial if they have control over their operation. Organizations must make an effort for continuous improvement in quality, efficiency, and cost reduction. Many organizations still follow inspection after the production for quality related issues. SPC helps companies to move towards prevention-based quality control instead of detection based quality controls. By monitoring SPC graphs, organizations can easily predict the behavior of the process. Statistical Process Control Benefits
Statistical Process Control ObjectiveSPC focuses on optimizing continuous improvement by using statistical tools to analyze data, make inferences about process behavior, and then make appropriate decisions. The basic assumption of SPC is that all processes are subject to variation. Variation measures how data are spread around the central tendency. Moreover, variation may be classified as one of two types, random or chance cause variation and assignable cause variation. Common Cause: A cause of variation in the process is due to chance, but not assignable to any factor. It is the variation that is inherent in the process. Process under the influence of common cause will always be stable and predictable. Assignable Cause: It is also known as “special cause”. The variation in a process that is not due to chance therefore can be identified and eliminated. Process under influence of special cause will not be stable and predictable. How to Perform SPC1.Identify the processes: Identify the key process that impacts the output of the product or the process that is very critical to the customer. For example, plate thickness impacts the product’s performance in a manufacturing company, then consider the plate manufacturing process. 2. Determine measurable attributes of the process: Identify the attributes that need to measure during the production. From the above example, consider the plate thickness as a measurable attribute. 3. Determine the measurement method and also perform Gage R&R: Create a measurement method work instructions or procedure including the measuring instrument. For example, consider thickness gage to measure the thickness and create an appropriate measuring procedure. Perform Gage Repeatability and Reproducibility (Gage R & R) to define the amount of variation in the measurement data due to the measurement system. 4. Develop a subgroup strategy and sampling plan: Determine the subgroup size based on the product’s criticality and determine the sampling size and frequency. For example collect 20 sets of plate thicknesses in a time sequence with a subgroup size of 4. 5. Collect the data and plot SPC chart: Collect the data as per sample size and select an appropriate SPC chart based on data type (Continuous or Discrete) and also subgroup size. For Example, plate thicknesses with a subgroup size of 4, select Xbar -R chart. 6. Describe natural variation of attributes: Calculate the control limits. From the above example, calculate the upper control limit (UCL) and lower control limit (LCL) for both Xbar Range. 7. Monitor process variation: Interpret the control chart and check whether any point is out of control and the pattern. Example: check Xbar R chart If the process is not in control, then identify the assignable cause(s) and address the issue. This is an ongoing process to monitor the process variation. Additional Statistical Process Control ResourcesControl limits are the voice of the process (different from specification limits, which are the customer’s voice.) They show what the process is doing and act as a guide for what it should be doing. Control limits also indicate that a process event or measurement is likely to fall within that limit. Control charts : A Control chart is one of the primary techniques of statistical process control (SPC). The control chart is a graphical display of quality characteristics that have been measured or computed from a sample versus the sample number or time. Furthermore, control chart contains a center line represents the average value of the quality characteristics and two other horizontal lines known as upper control limit (UCL) and lower control limit (LCL) Selection of an appropriate control chart is very important in control charts mapping. Otherwise, it ended up with inaccurate control limits for the data. The selection of control chart depends on the data type: Continuous or Discrete? Variable (Continuous) Control ChartsMeasure the output on a continuous scale. It is possible to measure the quality characteristics of a product.
Attribute(Discrete) Control Charts:The output is a decision or counting. It is not possible to measure the quality characteristics of a product. In other words, it is based on the visual inspection like good or bad, fail or pass, accept or reject.
Statistical Process Control LinksGreat decision matrix here: https://www.moresteam.com/toolbox/statistical-process-control-spc.cfm Statistical Process Control VideosSix Sigma Green Belt Statistical Process Control QuestionsQuestion: In the Control Phase of a LSS project a Belt will identify key metrics that can be monitored and analyzed to give an indication that a process may be moving towards an out of spec condition. When he applies this approach he is using __________________. (A) Poisson Derivatives Answer: Unlock Additional Members-only Content!To unlock additional content, please upgrade now to a full membership.Upgrade to a Full Membership If you are a member, you can log in here. Question: Statistical process control (SPC) is best defined as the use of (A) Pareto charts to understand and control a process Answer: Unlock Additional Members-only Content!To unlock additional content, please upgrade now to a full membership.Upgrade to a Full Membership If you are a member, you can log in here. Authors
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