CAPSIGMA Subcommand (SPCHART command)

This subcommand defines the capability sigma estimator, which is required in computing all the Process Capability Indices except K. There are four options:

RBAR. Mean sample range. The estimated capability sigma is based on the mean of the sample group ranges.

SBAR. Mean sample standard deviation. The estimated capability sigma is based on the mean of the sample group standard deviations.

MRBAR. Mean sample moving range. The estimated capability sigma is based on the mean of the sample moving ranges. The span that is defined by the SPAN subcommand is used. (Recall that its passive default value is 2.)

WITHIN. Sample within-group variance. The estimated capability sigma is the square root of the sample within-group variance.

The validity of specification depends on the chart specification (i.e., /XR, /XS, /I, /IR, or /CPCHART).

Table 1. Valid CAPSIGMA options by chart specification
Chart Specification Valid CAPSIGMA Options
XR RBAR (default), SBAR, WITHIN
XS RBAR, SBAR (default), WITHIN
I MRBAR (default)
IR MRBAR (default)
CPCHART RBAR, SBAR, WITHIN, MRBAR
  • When this subcommand is omitted or specified without a keyword by the user, the default conditional on the chart specification is implicitly assumed (see the table above). When the chart specification is /CPCHART, the default is RBAR when the data are organized in subgroups, and MRBAR otherwise.
  • An invalid specification (e.g., /I and /CAPSIGMA = RBAR) causes a syntax error when the chart specification is /XR, /XS, /I, or /IR and an applicable STATISTICS keyword is specified. An invalid specification (for those same chart types) without an applicable STATISTICS keyword generates a syntax warning.
  • When the chart specification is /CPCHART, an invalid specification (e.g., /CAPSIGMA = RBAR when the data are not organized in subgroups) causes a syntax error.
  • When the chart specification is /XR, /XS, /I, or /IR, CAPSIGMA is silently ignored if the subcommand specification is valid (with respect to the chart specification) but an applicable STATISTICS keyword is not specified.
  • CAPSIGMA is silently ignored if the chart specification is not /XR, /XS, /I, /IR, or /CPCHART.
  • A duplicated subcommand name causes a syntax error.
  • A duplicated keyword is silently ignored, but if two or more keywords are specified and they do not have identical meanings, a syntax error message is issued.