#### Capability Threshold

The Capability dialog box lets you set values that determine the process capability threshold for data used in the software's reports and charts. You also can specify whether or not the software will calculate capability indexes when the process is out of control, and whether or not Cp and Pp will be calculated for variables with unilateral tolerances.

To access this dialog box, choose Tools | Calculations. Then click Capability Threshold from the Calculation Specification dialog box.

From the Options tab, choose Calculation Settings, then click on Capability Threshold.

Capability dialog box

Cpk Threshold - Use this field to enter the value for Cpk, the actual capability index. This value is a measure of how many sigma half curves will fit between the process center and the closest sigma limit. Enter a numeric value (a typical target is 1.33, indicating the process center is four sigma away from specification limits) or simply select the desired number of sigma limits by clicking on the applicable button, and the system will insert the numerical value for you.

Ppk Threshold - Use this field to enter the value for Ppk, the potential capability index (uncentered). Enter a numeric value or select the number of sigma limits by clicking on the applicable button, and the system will insert the numerical value for you.

The threshold values are limits that determine whether or not the process should be judged "capable".

Calculate Capability when out of control - This check box determines whether or not the software will calculate and display capability indexes (Cp, Cpk, Cr, Pp, Ppk, and pr) when any of the selected control tests fail. Generally, the computation of capability indexes depends on the process being in control. However, you can select this option to always have the capability indexes calculated, even when the process is out of control.

Calculate Cp/Pp for unilateral tolerance -This check box controls the way the software handles the calculation of Cp and Pp for unilateral (one-sided) tolerance variables. Unilateral variables have a zero value for either the upper or lower tolerance. Select this option if you want Cp to be calculated in situations where no lower (or upper) tolerance exists. Do not select this option if you do not want Cp to be calculated. "NONE" will print on the report or chart instead of the Cp or Pp value.

For a unilateral tolerance like True Position or form, the software computes Cpk, only considering the real tolerance (so it computes Cpu and uses that, it doesn’t compute Cpl). Because of this you may see odd conditions where Cpk is larger than Cp which is not possible in normal circumstances. The software considers the variable to be unilateral when the nominal and lower tolerances are zero and the upper tolerance is a positive number. This is the case for the values where zero is a mathematical limit and not really a tolerance.

Capability Calculation Rules - These options let you choose the rules used for calculating capability. Using these options you can choose what the software calls Pp and Ppk as Cp and Cpk.

- Use estimated std.dev. - Cp and Cpk will use estimates of the standard deviation from the selected control chart (RBar/d2 for example).

- Use sample std.dev. - Pp and Ppk will use the sample standard deviation in the calculations.