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254 Part IV: Quality Assurance
is made to detect the cause and perpetuate it instead of eliminating it. In this manner,
the process can eventually be brought into a state of statistical control.
Dr. Shewhart, the inventor of control charts, recommended very strongly that
a process should not be judged to be in control unless the pattern of random variation
has persisted for some time and for a sizeable volume of output.
Note: If a control chart shows a process in statistical control this does
not mean that all special causes have been eliminated completely; rather it simply
means that for all practical purposes it is reasonable to assume or adopt a hypothesis
of common causes only.
Preparation for Use of Control Charts
Part IV.B
In order for control charts to serve their intended purpose, it is important to
take a few preparatory steps prior to the implementation of control charts. These
steps are:
1. Establish an environment suitable for action. Any statistical method will fail
unless management has prepared a responsive environment.
2. Define the process and determine the characteristics to be studied. The process
must be understood in terms of its relationship to the other operations/
users, and in terms of the process elements (for example, people,
equipment, materials, methods, and environment) that affect it at each
stage. Some of the techniques discussed earlier such as Pareto analysis
or the fishbone chart can help make these relationships visible. Once
the process is well understood then the next step is to determine which
characteristics are affecting the process, which characteristics should be
studied in depth, and which characteristics should be controlled.
3. Correlation between characteristics. For an efficient and effective study,
take advantage of the relationship between characteristics. If several
characteristics of an item tend to vary together, it may be sufficient
to chart only one of them. If there are some characteristics that are
negatively correlated, a deeper study is required before any corrective
action can be taken on such characteristics.
4. Define the measurement system. The characteristic must be operationally
defined so that the findings can be communicated to all concerned
in ways that have the same meaning today as they did yesterday.
This includes specifying what information is to be gathered, where,
how, and under what conditions. The operational definition is very
important for collecting data since it can impact the control charts
in many ways. Moreover, the analysis of data depends on how the
data are collected. Thus, it is extremely important that the data contain
the pertinent information and are valid (for example, appropriate
sampling schemes are used) so that its analysis and the interpretation
of the results is done appropriately. Moreover, one should always
keep in mind that each measurement system has its own inherent
variability. Thus, the accuracy of any measurement system is as
important as the elimination of the special causes affecting the process.