Purpose
Probability of a defect is used to assess the likelihood that a defect will occur. To determine the probability of defects, various statistical distributions can be used for both discrete and continuous data.
Anatomy
A. With continuous data the expected probability of defect is usually established by applying the normal distribution.
B. When discrete data is used we are merely counting the frequency in which something happens or fails to happen (go/no go, good/bad, yes/no) therefore a ratio is used to establish the observed probability of defect. The expected probability of defect can be obtained by discrete distribution such as Poisson and Binomial.
C. Probability of defect p(d) occurring below the lower specification limit.
D. Probability of defect p(d) occurring above the upper specification limit.
E. Probability of defect p(d) occurring above the spec limit.
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A Quality Control Plan is a documented description of the activities needed to control a process or product. The objective of a QCP is to minimize variation.
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The Weibull distribution is applicable to make population predictions around a wide variety of patterns of variation.