Control charts are used to determine whether process stability has been upset by special or assignable causes. Oracle Quality provides the following types of control charts and pairs of control charts.

• Xbar and R charts (XBar R): A pair of line graph charts which plot subgroups of data collected over a continuous period of time. The Xbar chart plots the average value for each subgroup. The R chart, or range chart, plots the range (difference between the maximum and minimum observations within a subgroup) of values within each subgroup.

• Individual X and Moving Range charts (XmR): A pair of charts based on individual data points instead of subgroups. The individual X chart simply plots individual data points. The moving range chart plots a specified range of the data points up to and including the current data point. For example, if a subgroup size of 5 is specified then the 10th point on the Moving Range chart is plotted using the range between points 6 and 10. These charts are commonly used when the process results do not fall into subgroups, for instance when the process is slow and data does not form clusters over a period of time.

• Xbar and S charts (XBar S): A pair of line charts similar to the Xbar and R charts except that the standard deviation is used instead of the range when the S chart is plotted. The S chart is an inherently superior estimator of process standard deviation than the R chart, for any sample size, because it uses all observations. The R chart uses only maximum and minimum observations. The difference in efficiencies of these two estimators become acute for large sample sizes. For example, if you had a subset size of 20 that contained one outlying data point, the range plotted on an R chart would be large even though the remainder of the data is fairly evenly distributed.

Subgroups

Selection of a subgroup is extremely important when defining a control chart. Improper subgroup selection can lead to misleading results and false corrective actions.

Creating and Viewing Control Charts

Control charts are Statistical Process Control (SPC) tools used for monitoring the state of a process. After viewing data samples, out-of-control processes can be adjusted accordingly. See: Statistical Process Control and Control Charts.

You can chart all or only specific quality results associated with a collection plan. You can create Control charts from "scratch" or you can create them by copying settings from another chart, descriptive statistic view, or custom report. Copying settings allows you to view the same subset of data in different ways. See: Copy Settings.

You can save your chart settings. You can re-create charts using these saved settings, and you can change these settings to create new charts.

You must define control limits before you can view a control chart. Control limits can be defined in any of the following three ways:

• Based on Collection Results: You can compute new control limits by entering the subgroup size and number of subgroups to use. Pressing the Compute New button then computes control limits based on the data most recently collected for the specified Collection Plan.

• Based on Saved Limits: You can use the control limits that were last used to generate the chart. You can also select saved limits from another chart, however, the selection of charts is limited by the chart type and the collection element.

  Note: You can only save one set of control limits for a chart.

• Based on Manual Entries: Enter the subgroup size, number of subgroups to use, and all control limits computed via some other mechanism.

   To create control charts using copied settings:

1. Navigate to the Control Charts window.

2. Choose the Copy Settings button. See: Copying Settings.

3. Enter missing information or change the copied information as required (See below).

   You can change all copied settings except the collection plan.

   To create control charts:

1. Navigate to the Control Charts window.

2. Enter the Chart Name.

   To save your chart parameters, you must enter a chart name.

3. Select the Collection Plan to chart.

   If you are creating a chart, you can select any collection plan, even those that are no longer effective. If you are changing a chart, you cannot change the collection plan.

4. Optionally, enter the Chart Title.

   The text you enter here is displayed at the top of the chart.

5. Optionally, enter the Chart Description

   The text you enter here is displayed at the top of the chart, under the chart title.

6. Select the Chart Type. See: Control Charts

   Recommendations for the use of each set of charts are as follows:

   XBar R: Use when the subgroup size is less than 10.

   X mR: Use when monitoring a process that produces quality results that do not naturally fall into subgroups.

   XBar S: Use when the subgroup size is greater than 10.

7. Select the Collection Element.

   This is the primary collection element that you want to analyze. It must have a numerical data type.

   Attention: Quality results are always grouped by occurrence or by the order in which the results were collected.

   To find and select quality results:

Navigate to the Show Results Where region of the Control Chart window. See: Finding Quality Results.

If you do not select which quality results to chart, all results associated with the collection plan are used.

   To define control limits based on collection results:

1. From the Control Charts window, select the Control Limits button.

2. Select or enter the Control Limit Name.

3. Enter the Description.

   You can use descriptions to identify the set of data you are charting.

4. Enter the Subgroup Size. You must enter a subgroup size and it must be between 2 and 25.

   Attention: You must recalculate control limits after changing the subgroup size.

5. Optionally, enter the Number of Subgroups to use in the calculation of control limits. If you do not enter the number of subgroups to use for the calculation, you can manually specify the control limits.

6. Choose the Compute New button.

   To choose saved control limits:

1. From the Control Charts window, select the Control Limits button.

2. Select the Control Limit Name of the saved set of control limits.

   You can also select saved limits from other charts, however, the selection of charts is limited by the chart type and the collection element.

3. Choose OK to save your work.

   To manually specify control limits:

1. Specify control limits by manually entering new limits - Upper X and R Control Limits, X and R Average values, and Lower X and R Control Limit values.

2. Choose OK to save your work.

   To view charts:

Choose the View Chart button. The control chart you created will display in a separate window.

   To save chart settings:

Choose the Save Settings button if you want to save the inquiry settings.

   To export chart results:

Choose Export Results from the Tools menu. See: Exporting Quality Results.