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WaveRIDER
®
Process Capability
A process capability index is a standard measure of how a process compares with its
specification limits—how a process
is
performing relative to how it
is supposed to
perform. As opposed to the control chart, which shows detailed information about how
the data compares with control limits, a capability index is a summary of how the data
compares with the specification limits.
Two common capability indicators are Cp and Cpk. These values are shown in the
Statistics Box on each SPC Worksheet.
For both of the index values, the data used to determine them is dictated by the
subgroup size (N) chosen by the user. In the case where N=1, individual data is used—
for N>1, average data is used (x bar).
The figures below give a graphical representation of the concept of Cp and Cpk. Notice
that in each graph, the same upper and lower specification limits (USL, LSL) are used.
The values of Cp and Cpk will differ according to the data that is compared with those
specifications.
Depending on the particular process being monitored, the desired value for Cp and Cpk
may differ. In general, however, a Cp and Cpk of 1.33 or above is desired. This
assures that the process is not only capable of meeting the required specification limits,
but also has a built-in margin for error that may be needed in special circumstances. In
addition to targeting a certain minimum Cp and Cpk, it is also desirable to have these
two values equal one another. This indicates that the process is well-centered between
the specification limits.
