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Statistical Process Control (SPC) (Page 4 of 5)
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Once the
control limits have been included on the control charts (also in the
form of horizontal lines like the specification limits), the operator
can start using the chart
visually
to detect anomalous
trends in the process that she would need to notify the engineer about.
Figure 3.
Example of a control chart showing data that are
slightly
off-centered, but nonetheless in control and within specs
For instance,
any measurement
outside
the control limits is an automatic cause for alarm, because the
probability of getting such a measurement is low. Four (4) or more
consecutively
increasing or decreasing points form a trend that is not normal, and
therefore deserves attention. Six (6)
consecutive
points on
one side
of the mean also deserve investigation. When such abnormalities are
observed, the process owner must take an action to bring the process
back to its normal behavior.
Control
limits must be
recomputed
regularly (say, every quarter), to ensure that the control limits being
used by the operator are reflective of the
current
process behavior.
Read more about:
Control Charting.
The
Process
Capability Indices
Being able to
monitor a process for out-of-control situations is one thing; knowing
how a process actually performs is another. Eyeballing
the centering and shape of a data distribution can give us quick, useful
information on how the corresponding process behaves, but it is not very
helpful in quantifying the process'
actual
and potential
performance. It is for this reason that statisticians have come up
with methods for expressing the behavior or capability of process
distributions in terms of single numbers known as
process capability indices.
Process
capability refers to the
ability of a process to meet
customer requirements or specification limits, i.e., how consistent its
output is in being within its lower and upper spec limits. A
process capability index should therefore be able to indicate how well
the process can meet its specs.
The
most basic process capability index is known as the simple process
capability index, denoted by 'Cp'.
Cp
quantifies the
stability of a process, i.e., the consistency of its output. As
mentioned earlier, the process capability indices discussed here presume
the
normality of the process. As such, the inconsistency of the
process may be measured in terms of the standard deviation or sigma of the output
data of the process. This is what Cp does - it uses the sigma to quantify the variation of a process,
and compares it against the distance between the upper spec limit (USL)
and lower spec limit (LSL) of the process. In mathematical form:
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