Control Charts Other Variable Control Charts Chapter Seven
Control Charts 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 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 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 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 All individual values Cluttered looking Useful when examining individual variation
Control Charts 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 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 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 Pre-control charts Study and compare product produced with tolerance limits.