6. Frequently asked questions
Can we assume that the normal law applies systematically?
The method described above is based on the assumption that the distribution of the monitored value follows a normal distribution. This is very often the case, since for the distribution to be normal it is sufficient for the quantity under study to be under the influence of numerous factors of variation or disturbance.
For the dimensions of a mechanical part, variable chip thickness on raw parts, T° variations (atmosphere - lubricant), defects in material homogeneity, defects in the positioning of parts and tools, machine deformation, parts, workpiece holders, etc.
Each cause of disturbance has an independent effect.
One cause does not have a preponderant effect over the others....
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fr.wikipedia.org Wikipedia article on normal distribution, with links to numerical tests to check whether a population follows a normal distribution.
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