Spec Lines on a control chart?

Matt SMatt S PQ Systems Employee

I often see spec lines displayed on an Individuals chart. The argument for adding them is that anyone can quickly determine if the measured value is in or out-of-spec. One argument against adding spec lines to an "X" chart is that the focus changes from process stability and signals to simply acceptability or not. What are some other reasons you think spec lines should not be displayed on an Individuals chart?


  • The only risk  here is that the spec line is no clearly distinct on the conrol limit and can be confused. I know that spec limits are supposed to be banished to the histogram, but when you are trying to explain common-cause varation and scrap rates to upper management in a management review meeting - you gotta have 'em.

  • Matt SMatt S PQ Systems Employee

    That reminds me of a saying: "Upper management: Can't live without them, Can't live with them." :) 

  • Matt - One of my favorite volumes is Tom Pyzdek's "The End of Management"
  • Matt, I generally don't like having specification limits on a control chart for the reason you mentioned. However, if a "power in charge" insisted, then I would train anyone who works with the charts or views them to visually review the difference between the control limits and the specification limits. Doing this would give an approximation of process capability, e.g., a control limit that is close to a specification limit would require close monitoring and perhaps increased subgroup sampling to prevent product being produced beyond a specification limit.
  • I like including Spec limits as a visual indication of capability (or lack there of) with processes in the early stages of control activities. The Spec limits can be described as a milestone for CI efforts, i.e. bring the process into control, then work on centering and spread reduction as appropriate to  bring Control limits within Spec limits. As a training tool for a new team, it keeps all these concepts on 1 chart. It is important to make Spec limits visually distinct from Control limits.
  • It has to be emphasized several times for the "variation-challlenged" who are in attendance ( and that can include people from a broad spectrum besides mgt) that  this applies only to the individual-moving range chart and not the X-bar / R ( as if they knew what that means). The individuals - moving range chart I find more useful than any other for allowing the process to tell us of its ability to meet specifications. Trying to discuss process capability without knowldege of variation by the audience, is like trying to explain gravity without a fundamental knowledge of physics.
  • With you all the way Wireman. We manufacture prescription eyewear, so use Individuals charts almost exclusively.
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