Relative Frequency Polygon
(A worked Problem: Procedure explained in more detail
in Chapter 3)
Note about Relative Frequency Polygons: Relative Frequency Polygons are used
for comparing 2 frequency distributions that
have different numbers of
scores (n's). You would never plot only
one Relative Frequency Polygon. You
would always have 2 lines on the graph.
This page will show you how to plot one
of the lines and then will demonstrate the
comparison with the other frequency
distribution.
Use the following frequency distribution of 100 IQ scores create a Relative Frequency Polygon:
Real Lim 
App Lim 
Freq 
Mid Pt 
Rel F 
Cum F 
C R F 
Cum % 








Use the following frequency distribution of 250 IQ scores create a Relative Frequency Polygon:
App Lim 
Freq 
Mid Pt 
Rel F 




I. To create a relative frequency polygon we use only the relative Frequency and the midpoint
II. We also need to create two extra intervals with zero relative frequency for each of
the
frequency distributions that we are going to
graph.
A. An extra interval at the top with
relative frequency = 0
This allows us to bring the graph back down to the X axis.
B. An extra interval at the bottom with
relative frequency = 0
This allows us to start the graph at the X axis.
III. Label the axes
A. Label the X axis
a. It is best to
label the X axis with the midpoints beginning with the midpoint
of that extra low
interval that you created. Remember to check that you have
covered the
midpoints of both frequency distributions.
b. Remember also to provide a label that tells the units (in this case it is IQ)
B. Label the Y axis
a. Begin at 0
and label up to the largest relative frequency in both of the distributions.
(It is very rare for a relative frequency to be greater than .5.)
b. Remember to
also put the label "Relative Frequency" next to the numeric labels.
IV. This next graph shows how to plot the first two points of the frequency
distribution at the top
of the page.
A. The first (extra interval) has a midpoint (X) = 72 and a relative frequency (Y) = 0.
B. The second point has a midpoint (X) = 77 and a relative frequency (Y) = .05.
V. Now we continue with the rest of the relative frequency distribution by
plotting points for the
rest of the class
intervals.
VI. Once all the points are plotted we then connect them with straight lines.
VII. Finally we plot the relative frequency graph for the other frequency distribution with n = 250.
Hint: Remember that Relative
Frequency Graphs always have two lines.
Hint: Remember that to plot a
Relative Frequency Polygon you must first generate the
two extra class
intervals.
Hint: Remember that all the points must be plotted at the midpoint
Hint: Remember to connect the points with straight lines.
Hint: Remember that without
the proper labels the entire graph is wrong.
This also means that
you must somehow distinguish between the two different lines.
Use different markers
or different colors for the different lines.
Copyright © 2004 by Mark W. Vernoy