SPC Concept

Posted by Wan Naza at 2:04 AM
  • All processes vary.
  • The aim of SPC is to minimise variation.
  • Variation is expressed in "sample standard deviations" (SD)
  • SD is a mathematical term, based on probability theory.
  • Consider the process of driving a car along a dry, straight, level road.
  • Look at your hands on the steering wheel - they move as you make slight adjustments.
If you draw a graph of the movements you will have a "normal distribution curve"

In a normal distribution, most of the time your hands are in the middle

(In the secret language of SPC the "middle" is called the "average" or "mean" or "x-bar")

But sometimes your hands are a little to the left, at other times they're a little to the right

And on rare occasions they're on the extreme left or extreme right.

Taken from

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