The number of units in your process output that you expect to violate the specification limits. This expectation is based on the cumulative distribution function (CDF) of the distribution you choose to model your data. The expected performance can be expressed in terms of nonconforming parts per million (PPM) or per hundred (percent).
For example, your company manufacturers drill bits that measure 50mm in length. Your customer requires an expected performance of 2000 PPM defective, maximum. You analyze the capability of this process with normal capability analysis, which calculates the following statistics of expected performance:
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Expected Within Performance |
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Expected Overall Performance |
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PPM < LSL |
1167.13 |
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PPM < LSL |
1755.06 |
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PPM > USL |
306.55 |
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PPM > USL |
509.58 |
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PPM Total |
1473.68 |
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PPM Total |
2264.64 |
The expected overall performance is the characteristic that the customer will experience because it accounts for the variation of the entire process output. Its PPM total exceeds the customers specifications of 2000 PPM, so your process in inadaquate. However, the expected within performance, which is based on within-subgroup standard deviation, does not violate the customer's specification. You resolve to reduce variation between subgroups to bring the unacceptable overall performance closer to the acceptable within-subgroup performance, which is also called the entitlement.
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Note |
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