Zone Chartoverview how to example data see also
Zone ChartStat > Control Charts > Variables Charts for Subgroups > Zone
A hybrid between an X (or individuals) chart and a CUSUM chart. It plots a cumulative score, based on "zones" at 1, 2, and 3 standard deviations from the center line. Zone charts are usually preferred over X or individuals charts because of their utter simplicity: a point is out of control simply, by default, if its score is greater than or equal to 8. Thus, you do not need to recognize the patterns associated with non-random behavior as on a Shewhart chart. This method is equivalent to four of the standard tests for special causes in an X or Individuals chart.
You can modify the zone chart weighting scheme to provide the sensitivity needed for a specific process. For example, you may want one set of weights during the initial SPC phase for detecting special causes. Then you may want to change the weights when the process is in control and it is more important to detect drift. See [6] for discussions of the various weighting schemes.
You can display a chart based on subgroup means or individual observations. With data in subgroups, the mean of the observations in each subgroup is calculated, then plotted on the chart. When subgroup sizes are unequal, Minitab generates a standardized zone chart. By default, the process standard deviation, s, is estimated using a pooled standard deviation. You can also base the estimate on the average of subgroup ranges or subgroup standard deviations, or enter a historical value for s.
With individual observations, a point is plotted for each observation. By default, s is estimated using MR / d2, the average of the moving range divided by an unbiasing constant. Moving ranges are artificial subgroups created from the individual measurements. The moving range is of length 2, since consecutive values have the greatest chance of being alike. You can also estimate s using the median of the moving range, change the length of the moving range, or enter a historical value for s.
To limit the number of plotted points, use Zone Options > Display.
For more information, see Control Charts Overview, Variables Control Charts for Subgroups Overview, and Comparing a Zone Chart with a Shewart Chart.
All observations for a chart are in one column: Choose if data are in one or more columns, then enter the columns.
Subgroup sizes: Enter a number or a column of subscripts. If the subgroups are not equal, each control limit is not a single straight line but varies with the subgroup size. If the subgroup sizes do not vary much, you may want to force the control limits to be constant by specifying a fixed subgroup size using Zone Options > Estimate.
Observations for a subgroup are in one row of columns: Choose if subgroups are arranged in rows across several columns, then enter the columns.