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IRCHART Statement |
An alternative method for creating an individual measurements chart
is to use the XCHART statement, which uses an estimate of based on moving ranges of two consecutive measurements when the
subgroup sample sizes are all equal to one. Note that the XCHART
statement displays the control limit legend n=1 to indicate
the common subgroup sample size, whereas the IRCHART statement
displays a legend that indicates the number of consecutive measurements
used to compute the moving ranges (the "pseudo subgroup sample
size").
Nelson (1982) explains that the reason for estimating the process
standard deviation from moving ranges of two consecutive
measurements rather than the sample standard deviation of the measurements
is that "the moving range of two minimizes inflationary
effects on the variability which are caused by trends and oscillations
that may be present." Nelson suggests that any moving range that
exceeds 3.5 times the average moving range should be removed from
the calculation of the average moving range.
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