Apply statistical process control (SPC) : How do you know whether a deviation in product characteristics is a "normal" variation of a statistical nature, or whether it is due to a process malfunction caused by an assignable cause that needs to be corrected?

How do you know whether a deviation in product characteristics is a "normal" variation of a statistical nature, or whether it is due to a process malfunction caused by an assignable cause that needs to be corrected?
Apply statistical process control (SPC)

Authors : Pascal VEDEL, Jean-Michel POU

Publication date: December 10, 2011 | Lire en français

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2. How do you know whether a deviation in product characteristics is a "normal" variation of a statistical nature, or whether it is due to a process malfunction caused by an assignable cause that needs to be corrected?

We invite you to consult the fact sheet on Statistical quality control: the concept of process centring and dispersion [FIC 0562] for :

remind you of the concepts of statistical dispersion of values, normal distribution and dispersion represented by the standard deviation;
review how to calculate the mean and standard deviation of a normal distribution.

In what follows, we'll assume that the distribution of...

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How do you know whether a deviation in product characteristics is a "normal" variation of a statistical nature, or whether it is due to a process malfunction caused by an assignable cause that needs to be corrected?