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Other Variable Control Charts

Other Variable Control Charts

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Other Variable Control Charts

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  1. Other Variable Control Charts Chapter Seven Control Charts

  2. Other chart options • What happens when there is an insufficient number of sample measurements to create x-bar and r charts? • What happens when only one sample is taken? • What other charting methods are available? Control Charts

  3. Available chart options • Individual and Moving-Range • Moving-Average and Moving-Range • All Individual Values • Median and Range charts • Run charts • Variable sub-group size • Pre-control charts Control Charts

  4. Individuals and Moving-Range • Used when measurements are single values or when the number of products produced is too small to form traditional charts. • Moving ranges are calculated by measuring the value-to-value differences. • Interpretation is the same - check trends, runs, patterns, limits Control Charts

  5. Moving-average and Moving-range • Plot moving-average • replace oldest value with newest value • sample size n = number of selected values • Moving-averages smooth out short-term variation • Lags occur, charts are less sensitive to change Control Charts

  6. All individual values • Cluttered looking • Useful when examining individual variation Control Charts

  7. Median and Range charts • Median of the data is calculated and charted • Ease of calculation is a trade-off with some loss of sensitivity. • Constructed and interpreted like X-bar/R charts Control Charts

  8. Run charts • Used to monitor process changes associated with a particular characteristic over time. • Either variables or attribute data. • X-axis shows time. • Y-axis shows attribute/variable value. Control Charts

  9. Charts for variable sub-group size • Sub-group size coefficients must be recalculated • Each subgroup with a different sample size will have its own control limits plotted. • Con: lots of calculations! Control Charts

  10. Pre-control charts • Study and compare product produced with tolerance limits. Control Charts