equal, the shape of the histogram looks similar to the graph, with relative fre-
quencies on the vertical axis.
To draw afrequency polygon, we first place a dot at the midpoint of the
upper base of each rectangular bar. The points are connected with straight
lines. At the ends, the points are connected to the midpoints of the previous
and succeeding intervals (these are make-up intervals with zero frequency,
where widths are the widths of the first and last intervals, respectively). A fre-
quency polygon so constructed is another way to portray graphically the dis-
tribution of a data set (Figure 2.3). The frequency polygon can also be shown
without the histogram on the same graph.
The frequency table and its graphical relatives, the histogram and the fre-
quency polygon, have a number of applications, as explained below; the first
leads to a research question and the second leads to a new analysis strategy.
- When data are homogeneous, the table and graphs usually show a
unimodal pattern with one peak in the middle part. A bimodal pattern
might indicate the possible influence or e¤ect of a certain hidden factor
or factors.
Example 2.4 Table 2.4 provides data on age and percentage saturation of bile
for 31 male patients. Using 10% intervals, the data set can be represented by a
histogram or a frequency polygon as shown in Figure 2.4. This picture shows
Figure 2.3 Distribution of weights of 57 children.64 DESCRIPTIVE METHODS FOR CONTINUOUS DATA