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Control Charts

# Control Charts - Sections 5.1 5.7 Page 1 Control Charts...

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Unformatted text preview: Sections 5.1- 5.7 Page 1 Control Charts Principles of Statistical Thinking h All work occurs in a system of interconnected processes, h Variation exists in all processes, and h Understanding and reducing variation are the keys to success. Control Charts h Processes include equipment, material, people, operational definitions and instructions, and environment. All of these contribute to variation in process output. h Statistical Process Control (SPC) is the use of statistical tools to control and improve a process. h Control Chart is an SPC tool that shows the variation in a process variable or attribute. h Attribute Data: data indicating whether an item conforms to some quality characteristic. (Eg: defective or not defective) o p-chart used to monitor percent defective s Based on binomial distribution o c-chart used to monitor number of defects per unit area s Based on poisson distribution h Variable Data: continuous measurement that is crucial to the performance of an item. Processes have two types of variation. h Common cause variation is the variation inherent in the process. It is experienced on an ongoing basis, affecting every unit. h Assignable cause variation arises due to some special circumstance outside the normal process. It occurs sporadically and affects only certain units. Example. Suppose we are sealing capacitors in a can. h The critical characteristic that ensures a proper seal is can height. The height should be 210 + 1 mm. h Lets sample the sealing process and see how it is running. h We take 5 sealed cans at a time and measure their heights. h Then we average these five measurements, calculate the range, and plot the gG . h Based on the central limit theorem, we expect the average can heights to follow an approximately normal distribution that will be centered in the same place as the distribution for individual can height measurements with a standard deviation of : h 99.73% of the s should fall between 3 Sections 5.1- 5.7 Page 2 30 20 10 210.5 210.0 209.5 Sample Number Sample Mean X-bar Chart for Ht(mm) Mean=210.0 UCL=210.4 LCL=209.5 Rational Subgroups h The group of five cans that is taken and measured is referred to as a rational subgroup or sample . It represents the process at a given moment in time. How is this different from a random sample? h We aim to have the within subgroup variability small and that assignable causes, if they are present, will have the largest effect in between subgroups, resulting in shifts in location....
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Control Charts - Sections 5.1 5.7 Page 1 Control Charts...

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