Total Quality Management - CII Institute of Logistics

Total Quality Management
The curve in Figure 4.8 also shows standard deviations. The sections of the
curve are labeled here with the symbol (sigma). This is the symbol for the standard
deviation of the normal curve. The standard deviation is calculated mathematically and
can be estimated in graphic form, but for now let's just say this is a number that describes
how the measurements cluster around the middle of the normal curve.
In Figure 4.8 a normal curve is compared to the specification; 2% of the pieces at each
end or 4% will fall outside the specification.
Specification
2% 14% 34% 34% 14% 2%
_ _ _ _ _ _ _
X-3 X-2 X-1 X X+1 X+2 X+3
Figure 4.8 Normal Curve
The frequency distribution and the histogram are used for the following purposes:
Evaluating or checking processes
Indicating the need to take corrective action
Measuring the effects of corrective actions
Comparing machine performances
Comparing materials
Comparing vendors
The frequency distribution and histogram are like a snapshot or a picture of a process at a
particular time. Because of the time required to gather and record measurements for a
frequency distribution, it is not practical to make more than a few distributions per day.
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