Between these two values, 0.25 and 0.74, 0.5 seems to be a convenient
number to use as the width; 0.25 is another choice but it would create
many intervals (15) for such a small data set. (Another alternative is to
express ages in months and not to deal with decimal numbers.)- Since the smallest number is 12.4, we may begin our intervals at 12.0,
leading to the following intervals:
12.0–12.4
12.5–12.9
13.0–13.4
13.5–13.9
14.0–14.4
14.5–14.9
15.0–15.4
15.5–15.9
16.0–16.4- Count the number of swimmers whose ages belong to each of the nine
intervals, and the frequencies, then obtain the frequency distribution
of menarchal age of 56 swimmers (Table 2.3), completed with the last
column for relative frequencies (expressed as percentages).
2.1.3 Histogram and the Frequency Polygon
A convenient way of displaying a frequency table is by means of a histogram
and/or a frequency polygon. Ahistogramis a diagram in which:
- The horizontal scale represents the value of the variable marked at inter-
val boundaries.
TABLE 2.3
Age (years) Frequency
Relative
Frequency (%)12.0–12.4 1 1.8
12.5–12.9 8 14.3
13.0–13.4 5 8.9
13.5–13.9 12 21.4
14.0–14.4 16 28.6
14.5–14.9 4 7.1
15.0–15.4 7 12.5
15.5–15.9 2 3.6
16.0–16.4 1 1.8
Total 56 100.0
62 DESCRIPTIVE METHODS FOR CONTINUOUS DATA