Download Cp and Cpk definitions

Cp and Cpk definitions:
Pp
Pp is a Process Performance index which measures the performance of the
process. This index compares the variation of the process to the allowable variation
that is set by the specification limits (USL and LSL).
Process Performance index, Pp, is similar to Process Capability index, Cp. Outside
of the context of Statistical Process Control (SPC) there is no difference between
Cp and Pp, and only the term Cp is used. When sample data is in view,  is
estimated by the familiar sample standard deviation that is technically denoted as s.
It is within the context of SPC that the distinction between C p and Pp becomes
important. The difference is in the estimate of  that is used. In SPC,
measurements are collected in subgroups.
For Cp,  is estimated by 2 / d2 , which is the average range of the subgroups
divided by a scale factor called d2 that is dependent on the subgroup size. For Pp,
is estimated by s. Since 3DCS does not record measurements in numerous small
subgroups, but rather one large group, Pp is more appropriate to use.

The Pp index tells us about the capability of a process but does not tell us where the
process lies with respect to its center.
When the range between the Upper Design Limit (UDL) and Lower Design Limit
(LDL) is the same as the 6 sigma spread, the Cp index will be 1.0.
The Pp index is calculated for a bilateral tolerance, there is no universally accepted
way of defining Pp Index for a unilateral tolerance.
Controlling Factors for Cp:
Design Specifications
Standard Deviation
Sources of the Deviation
Ppk
The Ppk indicator is a Process Performance index which tells us how well the
process is centered, or how distant the mean of the process is compared to its
specification limit. The Ppk index can never be higher than the Pp index value. If the
Ppk and Pp values are equal, the process is centered. If the Ppk value is 0, the mean
of the process is centered about one of its specification limits.
min = Minimum Value
 Mean Value
 = Standard Deviation
Controlling Factors for Ppk:
Design Specifications
Standard Deviation
Sources of the Deviation
Central tendency of Process