Control Chart Setup
Explanation
This activity is used to setup statistical process control (SPC) for a specific data point in a specific control
plan. The setup is used both when creating control charts manually and when they are created automatically using
Real Time SPC.
During the setup, a control chart type is chosen. There exists several control chart types.
- p Chart - Gives information about the proportion of nonconforming units.
- np Chart - Gives information about the number of nonconforming units.
- c Chart - Gives information about the number of nonconformities.
- u Chart - Gives information about the number of nonconformities per unit.
- Xbar and R Chart - Gives information about the average and the range of the result set of
each analysis selected in the graph.
- Xbar and S Chart - Gives information about the average and the standard deviation of the
result set of each analysis selected in the graph.
- X and MR Chart - Gives information about individual results and moving ranges for each
analysis selected in the graph.
The available control chart types that can be chosen is depending on the data type and the inspection type the
specific data point has.
- If the data type is Attribute and the inspection type is Percent the
available chart types are p Chart and u Chart.
- If the data type is Attribute and the inspection type is Quantity the
available chart types are np Chart, c Chart, p Chart and u Chart.
- If the data type is Variable and the inspection type is Percent the
available chart type is X and MR Chart.
- If the data type is Variable and the inspection type is Quantity the
available chart types are XBar and R Chart, XBar and S Chart and X and MR
Chart.
- If the data type is Categorical and the inspection type is Percent or
Quantity the available chart type is p Chart.
Control charts are used to analyze a process or its outputs and take appropriate actions to achieve and maintain
a state of statistical control. A process is in statistical control ("in control") when all special
causes of variation have been eliminated and only common causes remain, i.e., observed variation can be attributed
to a constant system of chance causes.
Several statistical tests (alarm rules) can be activated to determine whether the control chart is "in
control" or "out of control".
Out of Specification: An alarm rule violation will be triggered when a point in the control
chart is above Inner Max or Outer Max or below Inner Min or Outer Min.
- Above Upper Control Limit: An alarm rule violation will be triggered when a point in the
control chart is above the Upper Control Limit (UCL).
- Below Lower Control Limit: An alarm rule violation will be triggered when a point in the
control chart is below the Lower Control Limit (LCL).
- 2 out of 3 beyond 2 sigma: An alarm rule violation will be triggered when at least two out
of three successive points in the control chart on the same side of the centerline (CL) are beyond the 2 sigma
limit.
- 4 out of 5 beyond 1 sigma: An alarm rule violation will be triggered when at least four out
of five successive points in the control chart on the same side of the centerline (CL) are beyond the 1 sigma
limit.
- 15 at or within 1 sigma: An alarm rule violation will be triggered when 15 successive points
in the control chart are at or within 1 sigma.
- 8 outside 2 sigma: An alarm rule violation will be triggered when eight successive points in
the control chart are outside +2 or -2 sigma.
- 8 below or above the centerline: An alarm rule violation will be triggered when eight
successive points in the control chart are below or above the centerline
- Runs up/down of length 8 or greater: An alarm rule violation will be triggered when eight
successive points in the control chart are in ascending or descending order.
If an alarm rule violation occurs for an activated alarm rule, information about the alarm rule violation will
be displayed in the control chart.
It is also possible to decide if any specification limits should be displayed in the chart and how to calculate
the control chart control limits.
To set up SPC for a data point click Setup SPC Data on the Control Plan
Lines tab and enter the settings in the Variable SPC Setup dialog. To see
the default settings of the SPC setup, click Default Settings. To activate the control chart setup click
Save Settings.
Prerequisites
- A control plan with at least one control plan line must exist.
- Real Time SPC and/or SPC is enabled for the control plan line.
System Effects
As a result of this activity, the control chart type is determined for a specific data point in a specific
control plan. Decisions are also made about how to calculate control limits, whether to include specification
limits in the control chart, and whether one or many alarm rules should be activated.