Principles of Food Sanitation

monitoring food safety. The greatest advan-
tage of an SQC program is, that it enables
management to monitor an operation con-
tinuously and to make operating a closely
controlled production process.
Sample selection and sampling techniques
are the critical factors in any QC system.
Because only small amounts (usually less than
10 g) of a product are used in the final analy-
sis, it is imperative that this sample be repre-
sentative of the lot from which it was selected.
Statistical quality control, also referred to
asoperations research, operations analysis, or
reliability, is the use of scientific principles of
probability and statistics as a foundation for
decisions concerning the overall acceptabil-
ity of a product (Marriott et al., 1991). Its
use provides a formal set of procedures in
order to conclude what is important, and
how to perform appropriate evaluations.
Various statistical methods can determine
which outcomes are most probable and how
much confidence can be placed in decisions.

Central Tendency Measurements

Three measurements are commonly used
to describe data collected from a process or
lot. These are the arithmetic mean or aver-
age, mode or modal average, and median.
The mean is the sum of the individual obser-
vations divided by the total number of
observations. The mode is the value of
observations that occurs most frequently in a
data set. The median is the middle value
present in collected data. By using these val-
ues, the manufacturer can represent charac-
teristics of central tendencies of the
measurements taken. Table 8–1 illustrates

calculated values for the mean, mode, and

median from a collection of sample data.

Variability

There must be a uniformity and minimal

variation in microbial load or other charac-

teristics between the products manufactured.

Two measures of variation are the range and

standard deviation. Measuring variability by

means of the range is accomplished by sub-

tracting the lowest observation from the

highest.

R=Xmax−Xmin

From Table 8–1 the calculation would be:

R = 20 −11 = 9

Because the range is based on just two

observations, it does not provide a very accu-

rate picture of variation. As the number of

samples increases, the range tends to

increase because there is an increased chance

of selecting an extremely high or low sample

observation. The sandard deviation is a more

accurate measurement of how data are dis-

persed because it considers all the values in

the data set. The formula for calculating the

standard deviation is:

S ()( ) ( )

n

xx x x x x

1

n

2

2

(^22) f
= -+-++--
Although this formula is more compli-
cated than the range calculation, it can be
determined easily by using a personal com-
puter. As the standard deviation increases, it
reflects increased variability of the data. To
maintain uniformity, the standard deviation
should be kept to a minimum.
Displaying Data
It is beneficial to represent data in a fre-
quency table, especially when a large sample of
numbers must be analyzed. A frequency table
displays numerical classes that cover the data
range of sampling and list the frequency of

132 PRINCIPLES OFFOODSANITATION

Table 8–1Central Tendency Values

Data Mean Mode Median

11,12,14,14,16,17, 15.67 14 16
18,19,20