Normal Probability Plot
Plot data against a normal distribution reference line. If points follow the line, data is normally distributed. Essential before calculating CPK.
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Before calculating CPK, you need to verify data normality. The probability plot provides a visual check: if points follow the reference line, data is normal. Combined with the Anderson-Darling p-value, you can confidently proceed to CPK analysis.
25 diameter measurements: Mean = 10.12, StdDev = 0.24 A-D test: A²* = 0.189, p = 0.892 > 0.05 R² = 0.987 → Points closely follow reference line → Data is normally distributed, CPK calculation is valid.
FAQ
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Histogram + Normality
Pair with a histogram to check distribution shape — combining both gives higher confidence in normality.
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Confirm normality before calculating CPK; otherwise the index has no statistical meaning.
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For non-normal data, use a Weibull probability plot for reliability analysis.
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