Statistical Methods in Medical Research 4ed

26 Describing data
Frequency
20
18
16
14
12
10
8
6
4
2
0
10 20 30 40 50 60 70 80 90 100 110 120
Number of lesions
Fig. 2.7 Histogram representing the frequency distribution for a grouped discrete variable (Table 2.3).
Relative frequency
40
30
20
10
0
25
35
45 55 65 75
Age (years)
Fig. 2.8 Histogram representing the relative frequency distribution for a continuous variable (age of
1357 men with lung cancer, Table 2.4). Note that the variable shown here is exact age. The age at last
birthday is a discrete variable and would be represented by groups displaced half a year to the left from
those shown here; see p. 32.