Using Process Capability

Process capability indices are industrial-accepted calculations for comparing the process output to defined specification limits. For a normal distribution, the process output is defined as :

gv_7bdc_sqsy7ed1

standard deviations from the mean. For non-normal distributions, Quality determines the Best-Fit Pearson distribution and calculates equivalent 99.73 percent deviations (at 0.00135 and 0.99865).

The following table shows equations that relate to process capability:

Equation Statistic
gv_7bdb_sqsy7ecf

The process potential is the ratio of the process distribution to specification limits. It is the potential capability if the process was perfectly centered. This equation requires both upper and lower specifications.
gv_7bda_sqsy7ecd

This equation represents the actual process capability. These equations account for shifts in the process center. The

gv_7bd6_sqsy7ecb

is the lower of the

gv_7bd5_sqsy7ec9

or

gv_7bd4_sqsy7ec7

values, or worst-case capability. In the case of a unilateral specification, the

gv_7bd6_sqsy7ec5

is set to the calculated

gv_7bd5_sqsy7ec3

or

gv_7bd4_sqsy7ec1

value.
gv_7bd3_sqsy7ebf

The lower process capability represents the process's ability to perform at the LSL. This equation requires an LSL.
gv_7bd2_sqsy7ebd

The upper process capability represents the process's ability to perform at the USL. This equation requires a USL.
gv_7bd1_sqsy7ebb

where:

gv_7bd0_sqsy7eb9

The 90 percent confident Cpk is an adjusted Cpk based on a 90 percent confidence. The result is heavily affected by the sample size. The larger the sample size, the closer the computed value is to the actual Cpk.

See References.

gv_7bcf_sqsy7eb7

The capability ratio is the percentage that the process distribution consumes of the specification. This equation requires both upper and lower specifications.
gv_7bce_sqsy7eb5

where:

gv_7bcd_sqsy7eb3

is the area under the curve from the mean to the LSL.

The percent below specification is the estimated area under the curve to the left of the LSL. This equation requires an LSL.
gv_7bcc_sqsy7eb1

where:

gv_7bcb_sqsy7eaf

is the area under the curve from the mean to the LSL.

The percent above specification is the estimated area under the curve to the right of the USL. This equation requires a USL.
gv_7bca_sqsy7ead

The total percent out of specification is the total estimated area under the curve outside of the specification limits.