Principles of Food Sanitation

They are relatively small and are unpre-
dictable in occurrence. There is a certain
degree of chance-cause variation present.
Assignable-cause variation is just what the
name implies. Cause can be “assigned” to a
contributing factor, such as a difference in
microbial load of raw materials, process, and
machine aberration, environmental factors,
or operational characteristics of individuals
involved along the production line. This vari-
ation, once determined, is controlled through
appropriate corrective action. When a process
shows only variation due to chance causes,
it is “under control.” Quality control charts
were developed in order to differentiate
between the two types of variation and to
provide a method to determine whether a
system is under control. Figure 8–6 illus-
trates a typical control chart for a quality
characteristic. The y-axis represents the char-
acteristic of interest plotted against the
x-axis, which can be a sample number or time
interval. The center line represents the aver-
age or mean value of the quality trait estab-
lished by the manufactured product when
the process is under control. The two hori-
zontal lines above and below the center line

are labeled so that as long as the process is in

a state of control, all sample points should

fall between them. The variation of the

points within the control limits can be attrib-

uted to chance cause, and no action is

required. An exception to this rule would

apply if a substantial number of data points

fall above or below the center line instead of

being randomly scattered. This would indi-

cate a condition that is possibly out of con-

trol and would warrant further investigation.

If a point falls above or below the out-of-

bounds lines, one can assume that a factor

has been introduced that has placed the

process in an out-of-control state, and

appropriate action is required.

Control charts can be divided into two

types:

1. Control charts for measurement


2. Control charts for attributes

Measurement Control Charts

Measurement of variable control charts

can be applied to any characteristic that can

be measured. The Xchart is the most widely

used chart for monitoring central tendencies,

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Quality Measurement

Upper Control Limit

Center Line

Lower Control Limit

Figure 8–6Typical control chart.