for each point value of DD are then estimated or even guessed. Table 6.4 gives these
values.
The table function relation can be conveniently shown in a graphical form.
Figure 6.6 depicts the relationship defined in Table 6.4.
Figure 6.6 shows the table function as piecewise linear functions. This implies
that when the X axis variable takes a value different from the specified point values,
linear interpolation is carried out to estimate the corresponding Y axis value.
Graphical representation of a table function helps in two ways:
(1) it helps in defining the overall shape of the curve. For Fig. 6.6, it is known that as
delivery delay of products goes up, the order rate for the products goes down and
vice versa. Therefore, the slope of the curve around the normal value must be
negative. The normal point is made up of the normal delivery delay and the
normal order rate values (4 months,10,000 units). The shape of the curve around
the extreme values of the X-axis variable is often different from that around the
normal point. If in the past, the real system has never achieved such extreme
values, then no past data will be available. The only guiding parameters would
then be intuition and logic. For example, in the case of Fig. 6.6, it may be
Fig. 6.6 Customer order
rate (OR) vs. Delivery Delay
(DD) table relation
Table 6.4 Table function
relationship between OR
and DD
DD (months)
OR (units/month)
0
20,000
2
20,000
4
10,000
6
6000
8
5000
182
6 Infrastructure as a System