Enhanced Control Charts for Variable Data with Predictive Process Capability Statement

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Enhanced control charts for variable data charting with inclusion of a process capability predictive statement (when appropriate) can be accomplished with 30,000 foot-level reporting.

Traditionally x-bar and r charts are to be used when there are multiple continuous responses for time-series data subgroupings; however, this control charting approach has mathematical issues for many/most situations.  This issue is overcome with 30,000-foot-level report-outs.

The control limits for xbar r chart are calculated as a function of within subgroup process variability. If a process has between subgroup common cause variability, control limits for xbar r chart calculations can yield false out of control signals.

The ASQ  Quality Progress published article titled “Control Charting at the 30,000-foot-level” by Forrest Breyfogle addresses this issue and provides an alternative approach for calculating control limits. A PDF of this article is available below.

Control Charts for Variable Data Example: Control limits for xbar r chart

The following xbar r chart has many out of control signals

control charts for variable data for xbar r chart show out of control conditions

 

 

Control Charts for Variable Data Example: Control Limits for Individuals Chart

However, these out of control signals do not occur if only one subgroup sample from the same process data were plotted using individuals control limits.
control charts for variable data using individuals chart Sample Yields limits that differ from control limits xbar r chart

The mathematical explanation for this difference is shown in the PDF article; however, the basic reason is:

  • Variability within subgroups determine control limits for xbar r chart
  • Variability between subgroups determine control charts for a individuals control chart.

This PDF article below also describes how to creation a process capability index statement that is easy for others to understand.

 

Control Charts for Variable Data Example: Alternative control limits for xbar r chart

A 30,000-foot-level chart is an alternative where both subgroup means and standard deviations are tracked as individuals charts to determine process stability.  In addition, if a process has a recent region of stability, a prediction statement is provided at the bottom of the chart in words that are easy to understand.

General information about 30,000-foot-level reporting:

Application of 30,000-foot-level charting to an organizational enterprise scorecard, operational excellence and improvement system:

A no-charge Minitab add-in is available for the easy creation of 30,000-foot-level charts

How to create 30,000-foot-level charts for a variety of performance measurement situations is described in:

For more information about xbar r issues and process capability reporting, download the published The ASQ  Quality Progress November 2003 published article titled “Control Charting at the 30,000-foot-level” PDF article. 

 

Contact Us to set up a time to discuss with Forrest Breyfogle how your organization might gain much from 30,000 predictive performance reporting.