Page 248 - Applied statistics and probability for engineers

226 Chapter 6/Descriptive Statistics

Scatterplot of Quality vs. Color
20
19
18
17
Quality 16
15
14
13
FIGURE 6-19 12
Scatter diagram of
wine quality and 11 2 3 4 5 6 7 8
color from Table 6-5. Color

3.50 3.75 4.00 100 200 300 4 8 12 2.0 4.5 7.0
20

Quality 16
12
4.00

pH 3.75
3.50
300
200
Total so 2
100

FIGURE 6-20 12
Matrix of scatter Color Density 8
diagrams for the
wine quality data in 4
Table 6-5. Color

and Agriculture (1974, Vol. 25) by T.C. Somers and M.E. Evans. The authors reported quality
along with several other descriptive variables. We show only quality, pH, total SO (in ppm),
2
color density, and wine color for a sample of their wines.
Suppose that we wanted to graphically display the potential relationship between quality and
one of the other variables, say color. The scatter diagram is a useful way to do this. A scatter
diagram is constructed by plotting each pair of observations with one measurement in the pair
on the vertical axis of the graph and the other measurement in the pair on the horizontal axis.
Figure 6.19 is the scatter diagram of quality versus the descriptive variable color. Notice
that there is an apparent relationship between the two variables with wines of more intense
color generally having a higher quality rating.
A scatter diagram is an excellent exploratory tool and can be very useful in identifying poten-
tial relationships between two variables. Data in Figure 6-19 indicate that a linear relationship