Run chart vs. control chart?

Hanen AHanen A PQ Systems Employee
What's the difference between a run chart and a control chart?

Best Answers

  • Sam KSam K PQ Systems Employee
    Accepted Answer
    Both charts plot values for a single characteristic over time, but a run chart is a more simple chart. Control charts include calculated control limits that allow you to visualize variation, while run charts do not. 

    You might find this article from Steve Daum helpful, http://pq.systems/run-vs-control-charts. Steve explains: 
    A run chart is the simplest of charts. It is a single line plotting some value over time. A run chart can help you spot upward and downward trends and it can show you a general picture of a process.

    A control chart also plots a single line of data over time. However, control charts include upper and lower control limit lines with a centerline. These lines are calculated based on the data being plotted, and this allows you to answer more questions about the process. 
  • Accepted Answer
    Hanen A, I would suggest using a simple run chart to plot your data, then calculate your control limits when you have enough samples. Doing this lets you overlay the "new" control limits over the plotted data. Any data points outside the control limits indicates a lack of statistical control.

Answers

  • Individuals chart: Plot of measurements of a single characteristic in time order.
    Control chart: Individuals chart with control limits added.
    Compliance chart: Individuals chart with specification limits added.
    Are there specific names for charts that include both control and spec limits?  And are there different names for any of these charts if they plot subgroup averages rather than individual values?
  • Beth SBeth S PQ Systems Employee
    X-bar and Range charts are used to plot the subgroup averages. You can learn more about them on our web site http://www.pqsystems.com/qualityadvisor/DataAnalysisTools/x_bar_range.php. Charts that display both control and spec limits are called "misleading." Joking aside, we do often see spec lines displayed on Individuals charts. While it allows you to to quickly determine if the measured value is in or out-of-spec, adding spec lines to an "X" chart can change the focus from assessing process stability to assessing acceptability. What are other thoughts? Do you find spec lines on control charts helpful, or misleading?
  • I mostly find it important to know the difference.  If the process is in control and the spec limits are far wider than the control limits no worries or action is needed. If the spec limits are far wider than the control limits it might not even matter if the process is out of control.
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