Applied Statistics and Probability for Engineers

210 CHAPTER 6 RANDOM SAMPLING AND DATA DESCRIPTION

Figure 6-17 A digidot plot of the compressive strength data in Table 6-2.

Leaf Stem Time series plot

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

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5

7

189

8

7108

960934

0361410

8544162106

3073050879

471340886808

29583169

413535

103

580

15

7

7

6

For example, consider Fig. 6-16(a), which presents a time series plot of the annual sales

of a company for the last 10 years. The general impression from this display is that sales show

an upward trend.There is some variability about this trend, with some years’sales increasing

over those of the last year and some years’sales decreasing. Figure 6-16(b) shows the last

three years of sales reported by quarter. This plot clearly shows that the annual sales in this

business exhibit a cyclicvariability by quarter, with the first- and second-quarter sales being

generally greater than sales during the third and fourth quarters.

Sometimes it can be very helpful to combine a time series plot with some of the other

graphical displays that we have considered previously. J. Stuart Hunter (The American

Statistician, Vol. 42, 1988, p. 54) has suggested combining the stem-and-leaf plot with a time

series plot to form a digidot plot.

Figure 6-17 shows a digidot plot for the observations on compressive strength from

Table 6-2, assuming that these observations are recorded in the order in which they

1982 1983 1984 1985 1986 19871988 1989 1990 1991 Years

Sales,

x

Sales,

x

(a)(b)

1 2 3 4 1 2 3 4 1 2 3 4 Quarters

1989 1990 1991

Figure 6-16 Company sales by year (a) and by quarter (b).

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