- Create and interpret frequency polygons
- Create and interpret cumulative frequency polygons
- Create and interpret overlaid frequency polygons
Frequency polygons are a graphical
device for understanding the shapes of distributions. They serve
the same purpose as histograms, but are especially helpful in
comparing sets of data. Frequency polygons are also a good choice
for displaying cumulative frequency distributions.
To create a frequency polygon,
start just as for histograms, by choosing
a class interval. Then draw
an X-axis representing the values of the scores in your data.
Mark the middle of each class interval with a tick mark, and label
it with the middle value represented by the class. Draw the Y-axis
to indicate the frequency of each class. Place a point in the
middle of each class interval at the height corresponding to its
frequency. Finally, connect the points. You should include one
class interval below the lowest value in your data and one above
the highest value. The graph will then touch the X-axis on both
Frequency polygons are useful for comparing distributions.
This is achieved by overlaying the frequency polygons drawn for
different data sets. It is also possible to plot two cumulative
frequency distributions in the same graph.