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Chapter 18: A. Basic Statistics and Applications 231
10
Frequency
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Data
Figure 18.17 Frequency polygon for the data in Example 18.23.
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Relative frequency
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Figure 18.18 Relative frequency polygon for the data in Example 18.23.
Part IV.A.4
polygon and the relative frequency polygon for the data in Example 18.23 are
shown in Figure 18.17 and Figure 18.18 respectively.
Sometimes a data set consists of a very large number of observations, which
results in having a large number of classes of very small widths. In such cases
frequency polygons or relative frequency polygons become very smooth curves.
For example, Figure 18.19 shows one such smooth curve.
Such curves are usually called frequency distribution curves, which represent
the probability distributions of continuous random variables. Thus, histograms
eventually become the basis of probability distributions.
The shape of the frequency distribution curve depends on the shape of its corresponding
histogram that, in turn, depends on the given set of data. The shape of
a frequency distribution curve, in fact, can be of any type, but in general, there are
three types of frequency distribution curves as shown in Figure 18.20.