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Cc:
Predicted value of Cc. Six sigma quality requires a Cc ≤ 0.25 or
below. For a statistical tolerance, Cc is calculated assuming all the
inputs are exactly on target. As such, it will frequently be zero and
represent the best case. It tends to underestimate what will occur
in practice.
Cpk:
Predicted value of Cpk. Six sigma quality requires maintaining Cpk
above 1.5. The value shown assumes the inputs are exactly on
target so represents the best Cpk obtainable. Lower Cpk values will
result in practice as the inputs vary from their selected targets. To
maintain a Cpk of 1.5 requires an on target Cpk of 2 or more.
Defect Rate:
Predicted defect rate as either a percent defective or in defects per
million (dpm). 10,000 dpm is equivalent to 1% defective. 1000
dpm is equivalent to 0.1% defective. The defect rate is calculated
assuming the normal distribution. Six sigma quality requires 3.4
dpm or better. The value shown assumes the inputs are exactly on
target so represents the best defect rate obtainable. Higher defect
rates will result in practice as the inputs vary from their selected
targets. To average 3.4 dpm requires an on target defect rate that is
considerable lower.
Z-Score:
Predicted value of Z-score. Six sigma quality requires maintaining
a Z-score above 4.5. The value shown assumes the inputs are
exactly on target so represents the best Z-score obtainable. Lower
Z-scores will result in practice as the inputs vary from their
selected targets. To maintain a Z-score of 4.5 requires an on target
Z-score of 6 or more.
Sigma Level:
Predicted sigma level. For statistical tolerances, the sigma level is
the same as the Z-score.
An interval for individual values is displayed at the bottom of the Capability Study window.
This tolerance represents an interval containing almost all the individual units of product.
For a statistical tolerance it is calculated as follows:
Upper End Interval for Values = Average + 4.5 Standard Deviations
Lower End Interval for Values = Average - 4.5 Standard Deviations
The number 4.5 in these formulas is selected so that no more than 3.4 units per million will
fall outside each end of the interval. The value 4.5 can be changed using the Number
Standard Deviations for Interval for Values… menu item from the Options menu.
The graphic visually represents the relationship between the interval for individual values
and the specification limits. The width of the normal curve displayed always matches the
interval for individual values.
To perform statistical tolerancing VarTran derives an equation for the output’s average and
standard deviation. These equations can be viewed using the View Equation button at the
bottom of the Output Variable dialog box as described in Section 4.6.
Predicting Capability
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