Total Quality Management - CII Institute of Logistics

Total Quality Management
2. Where is the process centered?
The average of the histogram and the specification midpoint are close together;
therefore, the process is well centered. When the average of the histogram and the
specification midpoint are far apart, then the process setting needs some
adjustment.
3. Is the process capable of meeting the engineering specification?
The spread of the process falls within the specification limits; therefore, the
process is estimated to be capable of meeting the engineering specification.
Remember that the histogram is merely a snapshot of the process. If we take another set
of data at another time, the picture may be different. However, the information we have
on this process tells us that it is capable of meeting the engineering specification.
People in manufacturing often form opinions about the quality of the parts they are
making based on far less information than we had in the example above. It is much wiser
and safer to use a statistical technique, as we did, to see what the job is doing before we
decide how good or how bad the parts are. Remember the old saying, "One picture is
worth a thousand words". Few people can look at a set of jumbled-up data and see the
same things they can see after the data have been organized in pictorial form.
Once you begin to think statistically about your job, you will find that you naturally want
to get factual data and plot it in the appropriate form. With this method, you will make
good decisions based on facts instead of giving opinions based on measurements from
only a few pieces.
The process spread is determined by the curve it forms. When this curve is compared to
the specification, we can tell when the process is making parts outside the specification.
When the spread of the curve is wider than the specification or when one tail of the curve
goes outside of the specification, we can estimate the number of pieces that fall outside
the specification.
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