full2.gif (11401 bytes)
Frequency Polygon
(A worked Problem:  Procedure explained in more detail in Chapter 3)

Using the following frequency distribution of 100 IQ scores create a Frequency Polygon:

Real Lim

App Lim

 Freq

Mid Pt

Rel F

Cum F

C R F

Cum %


134.5-139.5
129.5-134.5
124.5-129.5
119.5-124.5
114.5-119.5
109.5-114.5
104.5-109.5
99.5-104.5
94.5-99.5
89.5-94.5
84.5-89.5
79.5-84.5
74.5-79.5
69.5-74.5


135-139
130-134
125-129
120-124
115-119
110-114
105-109
100-104
95-99
90-94
85-89
80-84
75-79
70-74


0
5
5
5
10
10
15
15
10
10
5
5
5
0


137
132
127
122
117
112
107
102
97
92
87
82
77
72


0
.05
.05
.05
.10
.10
.15
.15
.10
.10
.05
.05
.05
0


100
100
95
90
85
75
65
50
35
25
15
10
5
0


1.00
1.00
.95
.90
.85
.75
.65
.50
.35
.25
.15
.10
.05
0


100
100
95
90
85
75
65
50
35
25
15
10
5
0

I. To create a frequency polygon we use only the frequency and the midpoint

II. We need to create two extra class intervals

    A.  An extra interval at the top with frequency = 0
            This allows us to bring the graph back down to the X axis.

    B.  An extra interval at the bottom with 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.

        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 if you can and label up to the largest frequency

        b.  Remember to also put the label "Frequency" next to the numeric labels
fp1.gif (5994 bytes)

IV.  This next graph shows how to plot the first two points.

    A.  The first (extra interval) has a midpoint (X) = 72 and a frequency (Y) = 0.

    B.  The second point has a midpoint (X) = 77 and a frequency (Y) = 5.

fp2.gif (6024 bytes)

V.  Now we continue with the rest of the frequency distribution by plotting points for the rest of the class intervals.

fp3.gif (6185 bytes)

VI.  Once all the points are plotted we then connect them with straight lines.

fp4.gif (8550 bytes)

Hint:  Remember that to plot a 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.

  Copyright © 2004 by Mark W. Vernoy