When should I use historical parameters?

If you previously made a control chart of your process, then you have known process parameters you can use to build a control chart. You can use historical parameter values with a control chart to establish a benchmark where the process was in the past. If the current data are out of control on a control chart that uses historical parameters, the out-of-control signal usually means that the process changed. If the current data are in control, the process has remained stable since you made the earlier control chart.

Example of determining historical parameters

To view the parameter estimates for a control chart, you can store them in the worksheet. For example, suppose you want to create an Xbar chart for data in column C1 and your subgroup size is 5. Do the following to store the mean and standard deviation estimates in the next available blank columns in the worksheet.

  1. Choose Stat > Control Charts > Variables Charts for Subgroups > Xbar.
  2. Choose All observations for a chart are in one column, then enter C1.
  3. In Subgroup sizes, enter 5.
  4. Click Xbar Options, then click the Storage tab.
  5. Under Store these estimates for each chart, select Means and Standard deviations.
  6. Click OK in each dialog box.

Example of using historical parameters

Suppose you want to use the historical mean and standard deviation to create a new Xbar chart:

  1. Choose Stat > Control Charts > Variables Charts for Subgroups > Xbar.
  2. Complete the dialog box as usual for the new data.
  3. Click Xbar Options.
  4. On the Parameters tab, enter the mean and standard deviation that were stored in the worksheet.
  5. Click OK in each dialog box.
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