Variability Basics

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Variability Basics

• By ???
• ?????????

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What is variability ?

• A formal definition
• the quality of nonuniformity of a class of
  etities.
• EX failure/repaire time?process
  time?temperature.

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Type

• Controllable Variation
• Random Variation

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Controllable Variation

• It occurs as a direct result of decision.
• Ex physical dimension ?time to manufacture.

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Random Variation

• It is a consequence of events beyond our
  immediate control.
• Ex the times between customer demands.

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• Variability is closely associated with randomness.

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The Roots of Randomness

• One interpretation of randomness is that because
  we have imperfect information.
• A second is that the universe actually be haves
  randomly.

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Robust policy

• It is one that works well most of the time.
• Robust policies differ from Optimal policies.

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Process Time Variability

• The random variable of primary interest in
  factory physis is the effective process time of a
  job at a work station.
• EX B(waiting) --gt A

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Causes of Variability

• Natural variability
• Preemptive Outages
• Nonpreemptive Outages
• Recycle

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Natural variability

• It is the variability inerent in natural process
  time ,which excludes random downtimes?setups?or
  any other external influence.

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Preemptive Outages (Breakdowns)

• This is the single largest cause of
  variability,because they occur whether we want
  them to or not.

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Nonpreemptive Outages

• They represent downtimes that will inevitably
  occur but for which we have some control as to
  exactly when.
• These typically occur between jobs , rather than
  during them.

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Recycle

• A major source of variability in manufacturing
  systems is quality problems.
• getting the job right

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Flow Variability

• Variability at one station can affect the
  behavior of other stations in a line by means of
  another type of variability.

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Conclusion

• Variability is a fact of life.
• Flexibility reduces the amount of variability
  buffering required in a production.

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Reference

• Factory Physis Wallace.j.Hopp