Remember, useful limits can be constructed with
as few as five or six consecutive values. However, the more data used to compute the
limits, the greater the certainty of the results.
Another way to spot trends is to look at the data points along the center line. If
eight or more consecutive data points are clustered on the same side of the center
line, a shift in the original baseline or performance of the process has probably
occurred, even without a data point falling outside the limits. This is a signal to be
investigated.
Also look for 6 consecutive points either increasing or decreasing in value. These
points could signal a trend. Other points to notice are 14 points in a row alternating
up and down; or 15 points clustered around the center line, both above and below
the line.
Notice that we don??™t say, ???This is a problem.??? We don??™t know yet. We do know
that something has happened that needs more focus.
c-Charts
While XmR charts are the most often applied in organizations and are the most
appropriate charts to use most often, they are not infallible.
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