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Interpreting
a Cu Sum Chart
A V-mask is used to determine
whether the process mean has drifted from the target. Subgroups with missing
data are not included in the analysis (i.e. the subgroup sample size, n,
must be constant for all subgroups). The performance of the control chart
is influenced by the design of the V-mask, which are used to define the
CuSum control limits . The
design parameters of the V-mask are the angle (q), which sets the size of
the V, and the distance (d), which sets the location of the vertex of the
V from the current subgroup. The user influences the value of these parameters
by specifying:
-
alpha (a): the probability of incorrectly concluding that a shift
in the process mean has occurred (i.e. a false alarm),
-
beta (b): the probability of failing to detect a shift in the process
mean, and
- Amount of Concern:
the shift in the process mean that the user desires to detect. Delta
must be expressed in the same units as the measurements.
An interesting property
of the Cu Sum's V-mask is its ability to detect when a shift occurred. With
any control chart, if you are collecting and analyzing data and a process
shift occurs, it may be several groups before the shift is detected as a
group out of control. The Cu Sum chart's V-mask will tend to indicate when
the out of control condition originated. For example, if the shift really
begins at group 40, the first out of control condition may occur when group
45 is collected, but the Cu Sum chart may indicate at that time that group
40 is out of control.
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