Inherent or Natural Variation

Inherent or Natural Variation
Due to the cumulative effect of many small unavoidable causes
A process operating with only chance causes of variation present is said to be “in statistical control”

Special or Assignable Variation

  • Due to

a)  improperly adjusted machine
b)  operator error
c)  defective raw material

  • A process operating in the presence of assignable causes of variation is said to be “out-of-control”
Understanding Variation

Understanding Variation

Sources of Variation

  • within unit(positional variation)
  • between units(unit-unit variation)
  • between lots(lot-lot variation)
  • between lines(line-line variation)
  • across time(time-time variation)
  • measurement error(repeatability & reproducibility)

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