Measurement of Reliability

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Measurement of Reliability

• Henry C. Co
• Technology and Operations Management,
• California Polytechnic and State University

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Measure of Reliability

• Failure Rate ? number of failures within a
  given time period.
• MTBF (mean time between failure) the amount of
  time between failure of a repairable product
  1/?.
• MTF (mean time to failure) the amount of time
  between failure of a non-repairable (i.e., must
  be replace) product 1/?.

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The Bath Tub Function
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How is ? measured?

• In order to determine the failure rate, we
  collect a random sample of components and
  determine the time of failure for each component
  and determine the estimated failure rate using
  the following equation
• ? (number of failures within the testing time
  period) ? ( total testing time period for all
  components in the sample)

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Illustration

• 14 random sample of light bulbs

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What is the failure rate for the testing period
of 450 hours?What is the MTF?

• Samples 1, 2, 3, 4 failed within 450 hours.
• The other 10 survived.

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• (number of failures within the testing time
  period) ? ( total testing time period for all
  components in the sample)
• (4) ? (359010015010450) 0.000821
  failures/hour.
• MTF 1/? 1/0.000821 1,218 hours.

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What is the failure rate for the testing period
of 900 hours?What is the MTF?

• Samples 1, 2, 3, 4 5 failed within 900 hours.
• The other 9 survived.

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• (number of failures within the testing time
  period) ? ( total testing time period for all
  components in the sample)
• (5) ? (35901001507009900) 0.000545
  failures/hour.
• MTF 1/? 1/0.000545 1,835 hours.

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What is the failure rate for the testing period
of 1350 hours?What is the MTF?

• Only sample 14 survived.

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• (number of failures within the testing time
  period) ? ( total testing time period for all
  components in the sample)
• (13) ? (3590100150700 1300 11350)
  0.001087 failures/hour.
• MTF 1/? 1/0.000545 920 hours.

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Why did we use the MTF and not the MTBF?
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