How to Set-up a Control Chart?

1.Select appropriate type of control chart to be used
2.Gather data to establish the control chart.

  • A minimum of 30 subgroups is required over a time frame as determined by the sampling plan.

3.Plot the data in time order on a Trend Chart.
4.Compute the control limits & plot them on the trend chart
5.Outliers identification & exclusion

  • Exclude the Out-of Control (OOC) points or outliers for which there are verified/confirmed special causes from the chart
  • Re-compute the control limits, excluding the OOC points
  • If there are fewer than 30 points remaining at any time, collect more data.  It’s very important that the control limits are calculated using at least 30 subgroups.

Note:Refer to Appendix A for Control Charts for Limited Production, i.e. < 30 subgroups.

6.Validate the computed control limits against data collected by re-plotting the control chart with data & new control limits

  • Do the limits detect known problems?
  • Are the limits too sensitive?  Would they flag problems you do not know how to react to?

7.Use the control limits established to monitor the critical parameter identified
8.For each parameter, every machine should have a separate control chart with separately computed control limits

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