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Confidence Statement Relative to Spec Limits
A confidence statement relative to spec limits is statement like: "With 95% confidence more than 99.87% are in spec." Such a statement results from passing a variables sampling plan. Based on the Pp and Ppk values, a variables sampling plan is identified that would pass and the resulting confidence statement constructed. Confidence statements relative to spec limits are displayed in Tabs 1 and 2 of the Test Distribution window.
Confidence statements relative to spec limits assume the underlying data fits the normal distribution. Before using one, you should pass a normality test. By default, the confidence statement relative to the spec limits is only displayed if one of the normality tests passes. For data that fits some other distribution, the data is first transformed along with the spec limits. Pp, Ppk and the confidence statement relative to the spec limits are then calculated using the transformed values.
An alternate approach is to use normal tolerance intervals.