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PROCESS
CAPABILITY CHARTS
Process Capability Charts
may actually be a misnomer. The graphics presented with this feature include
a histogram of the data, with a distribution curve overlaid. The statistics
provided largely deal with the capability indices. Capability indices attempt
to indicate, in a single number, whether a process can consistently meet
the requirements imposed on the process by internal or external customers.
Much has been written about the dangers of these estimates, and users should
interpret capability only after understanding the inherent limitations of
the specific index being used.
See also:
When
to Use a Process Capability Chart
Interpreting
a Process Capability Chart
Calculations:
Cp
Cr
Cpk
Cpm
Process
Performance
For a general discussion
of capability indices, histograms, and distributions, refer to any Statistical
Quality Control textbook, such as (Pyzdek, 1990, 1992a) or (Montgomery,
1991). For more information on the Johnson translation system, refer to
Johnson (1949, 1983) and Pyzdek (1991a, 1992b). For information concerning
the use of the Rayleigh distribution for True Position applications, see
Pyzdek (1991b, 1992a, 1992b).
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