Page 215 - The Handbook for Quality Management a Complete Guide to Operational Excellence

202 P r o c e s s C o n t r o l Q u a n t i f y i n g P r o c e s s Va r i a t i o n 203

charts will be used for process performance measures assessed
on a continuous scale. Also considered will be the skill level of
the personnel involved, need for sensitivity, and other resources
required to collect, record, and analyze the data.
7. Gather and analyze the data. Use one of the control charts described
in this chapter, plus common sense. It is usually advisable to have
at least two people go over the data analysis to catch inadvertent
errors in transcribing data or performing the analysis.
8. Track down and remove special causes. A special cause of variation may
be obvious, or it may take months of investigation to find it. The
effect of the special cause may be good or bad. Removing a special
cause that has a bad effect usually involves eliminating the cause
itself. For example, if poorly trained operators are causing variability,
the special cause is the training system (not the oper ator), and it is
eliminated by developing an improved training system or a process
that requires less training. However, the removal of a ben eficial
special cause may actually involve incorporating the special cause
into the normal operating procedure. For example, if it is discovered
that materials with a particular chemistry produce better product,
the special cause is the newly discovered material and it can be
made a common cause simply by changing the specification to
ensure that the new chemistry is always used.
9. Estimate the process capability. One point cannot be overemphasized:
the process capability cannot be estimated until a state of statistical
control has been achieved! After this stage has been reached, the
methods described later in this section may be used. After the
numerical estimate of process capability has been arrived at it
must be compared to management’s goals for the process, or it
can be used as an input into economic models. Deming’s all-or -
none rules (see Acceptance Sampling) provide a simple model
that can be used to determine if the output from a process should
be sorted 100 percent or shipped as is.
10. Establish a plan for continuous process improvement. Once a stable
process state has been attained, steps should be taken to maintain
it and improve upon it. SPC is just one means of doing this. Far
more important than the particular approach taken is a company
environment that makes continuous improvement a normal part
of the daily routine of everyone.

Statistical Analysis of Process Capability Data
This section presents several methods of analyzing the data obtained from
a process capability study.

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