Implement sampling procedures

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Inspection Levels

• 4 special inspection levels - S1, S2, S3, S4.
• 3 general inspection levels - I, II, III.
• Special inspection levels used when sample size
• must be kept small and larger sampling risks can
• be tolerated.
• Level II will be used unless another inspection
• level is specified.
• Level I is used when less discrimination is
• required, Level III when greater discrimination
  is
• required.

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Sample size code letters
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Sample size code letters (cont)
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Single Sampling Plans
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Double Sampling Plans

• Normal Inspection

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OC Curves

• Operating Characteristic Curves
• OC curve is a graph showing what any particular
• sampling plan can be expected to do in terms of
• accepting and rejecting batches.
• An understanding of the implications of an OC
• curve helps understand the risks to the
• manufacturer, consumer, and in deciding
• inspection levels and batch sizes.
• Each possible plan has its own OC curve.
• Horizontal scale - shows the percentage
  defective.
• Vertical scale - shows the percentage of batches
• that may be expected to be accepted if batches
  are
• produced with that percentage of defects.

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Using an OC curve

• Interpret the curve according to this example
• If the lot quality is 0.093 fraction defective,
  then the probability of acceptance, Pa, is 0.05.
• If the lot quality is 0.018 fraction defective,
  then the probability of acceptance, Pa, is 0.95.

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Alternative OC for Sample plans

• You can use oc-curves to compare alternative
  plans.
• Choose between the plans by their relative
  ability to detect rejectable lots.
• You should expect that the steeper the curve, the
  larger the sample size.

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OC Curves
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OC Curve Example

• Code Letter M
• AQL 1.5
• Sample size - 315
• Accept 10, Reject 11
• What happens if a batch with 3 defectives is
• submitted ?
• Find 3 on the X axis, and follow a vertical line
• up until it meets the 1.5 curve.
• Take a horizontal line across to the Y axis, and
• read off the value of 65.
• I.e., 65 of batches would be accepted and 35
• rejected.

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More OC curve examples
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Statistical Quality Control Online

• http//iew3.technion.ac.il/sqconline/milstd105.htm
  l
• This application gives the single and double
  sampling plans for attributes, according to the
  Military Standard 105E tables, for a given lot
  size and AQL

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Example of Online Calculation
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Result Single sampling
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The Double samplingResults
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Even gives the OC curve

• The OC curve describes the probability of
  accepting a lot, OC(p), as a function of the
  proportion non-conforming (p),

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Concrete examples

• EN 455 Part 1 for medical gloves - Pinholes
• General inspection level 1, AQL 1.5.
• Minimum sample size code letter L (200
• gloves)
• If a batch with 3 pinholes is submitted,
• there is a 75 chance of them being accepted.
• All Wales Universal Container contract
• Agreed AQL of 0.65, general Inspection
• level I.
• Sample code L (200 samples).
• If a batch with 5 leakers is submitted, there
• is a 99 chance of them being rejected.