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SPC Trick 8, Use Ranges to Find the Control Limits - A Wayworld Tutorial
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At this trick, (if you we're fishing, you'd be learning to cast the bait or fly now) we must know: · 
  • How do you calculate the Ranges between measurements? (The range is the distance between two numbers). · 
  • How does a Control Chart "stack into" a Normal Distribution? (See Trick Seven) · 
  • What is the "68, 95, 99.7 Rule"? (It tells about how tight or loose the process is).

When we get a very high or very low readings on our control chart should we worry about it, or is that reading a normal part of the process variation?

If the reading violates the "68, 95, 99.7 Rule", that is, if the odds are really low that the reading would have occurred, then we worry, especially if we have more than a few of these violations. But where do we draw the line on the Control Chart to apply the "68, 95, 99.7 Rule"?

This is a real trick. We use the Ranges, a math formula and a lookup table (not given in this tutorial) to find out where to draw the Upper Control Limit and the Lower Control Limit on the Control Chart. Any points outside these  Control Limits violate the "68, 95, 99.7 Rule".

Practice:
Here's the trick. Follow it like you're making a chocolate cake:
  • 1. Collect at least 20 points on your control chart. 
  • 2. Calculate the Average, and draw it on the control chart. (trick two) 
  • 3. Compute the Ranges between all the measurements. (trick six) 
  • 4. Compute the Middle Range. (the median range, trick six) 
  • 5. Compute the Upper Control Limit (UCL) and Lower Control Limit (LCL) with the following formula, and draw each of them on the Control Chart. 
    • UCL = Average plus (3.14 times the Median Range) 
    • LCL = Average minus (3.14 times the Median Range)

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