## Interpreting Charts for Individual Measurements and Moving Ranges

Montgomery (1996) points out that a moving range chart should be
interpreted with care because "the moving ranges are correlated,
and this correlation may often induce a pattern or runs or cycles
on the chart." For this reason Nelson (1982) recommends against
plotting the moving ranges. Nelson notes that the assumption
of normality is more critical for an individual measurements chart
than for an chart. You can use the NOCHART2 option in
the IRCHART statement to specify that only the individual
measurements chart is to be displayed. See Example 34.3
for an illustration. If, instead, you specify
the SEPARATE option, the charts for individual measurements
and moving ranges are displayed on separate screens.
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.

Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.