The normal or Gaussian distribution puts about 68.3 percent of values within 1 standard deviation of the mean, 95.4 percent within 2 and 99.73 percent within 3. Control limits, capability indices and most significance tests assume it, either directly or through averages. Real process data are often skewed or bounded, such as particle counts or flatness that cannot go below zero, so a normal probability plot comes before any ppm prediction.
Leakage is lognormal, not normal. Your Cpk of 1.5 is predicting ppm off a tail that doesn't exist.
Also heard as
- Gaussian distribution
- bell curve
Related terms
Standard deviation
Typical distance of individual values from their mean, the basic measure of spread, written as sigma or s.
Central limit theorem
Result that averages of many independent values tend toward a normal distribution even when the values themselves are not normal.
Process capability
How well a stable process's natural spread fits inside the specification limits, usually expressed as an index.
Residual
Difference between an observed value and the value a fitted model predicts for it.