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Probability Charts

Probability Charts. What is it?. Attributes control chart used with data collected in subgroups of varying sizes Shows how a process changes over time Control chart used to monitor the proportion of nonconforming units in a sample Only accommodates pass/fail type inspection

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Probability Charts

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  1. Probability Charts

  2. What is it? • Attributes control chart used with data collected in subgroups of varying sizes • Shows how a process changes over time • Control chart used to monitor the proportion of nonconforming units in a sample • Only accommodates pass/fail type inspection • Specifications before plotting

  3. When To Use It • Use p-charts when you can answer yes to these questions: • Do you need to assess system stability? • Is the data a count of nonconforming items per subgroup? • Can the counts be converted to proportions? • Are there only two outcomes to any given check? • Is the time order of subgroups preserved?

  4. Assumptions • Binomial Distribution • Probability of nonconformity is the same for all units • Each unit is independent • The inspection procedure is same for each sample and is carried out consistently from sample to sample

  5. Control Limits • P is the estimate of the long-term process mean established during control-chart setup • Lower Control Limits • Value of P=Standard value • Pros/Cons

  6. Potential Downfalls • Ensuring enough observations are taken for each sample • Accounting for differences in the number of observations from sample to sample

  7. Adequate Sample Size • Things to consider: • 100% inspection of processes • 50% chance of detecting a process shift

  8. Example:

  9. POM Example • Samples= 10 • Sample Size= 40 • Number of defects per sample:

  10. Questions

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