Frequency Polygon (1 of 2)

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A frequency polygon is a graphical display of a frequency table. The intervals are shown on the X-axis and the number of scores in each interval is represented by the height of a point located above the middle of the interval. The points are connected so that together with the X-axis they form a polygon.

A frequency table and a relative frequency polygon for response times in a study on weapons and aggression are shown below. The times are in hundredths of a second.

Lower
Limit
Upper Limit
Count
Cumulative
Count
Per Cent
Cumulative
Per Cent

25
30
35
40
45
50
30
35
40
45
50
55
1
4
8
15
3
1
1
5
13
28
31
32
3.12
12.48
24.96
46.80
9.36
3.12

3.12
15.62
40.62
87.50
96.88
100.00
Note: Values in each category are > the lower limit and ≤ to the upper limit.


Frequency polygons are useful for comparing distributions.
This is achieved by overlaying the frequency polygons drawn for
different data sets. The figure below provides an example. The data come
from a task in which the goal is to move a computer mouse to a
target on the screen as fast as possible. On 20 of the trials,
the target was a small rectangle; on the other 20, the target
was a large rectangle. Time to reach the target was recorded on
each trial. The two distributions (one for each target) are plotted
together. The figure shows that although there is
some overlap in times, it generally took longer to move the mouse
to the small target than to the large one.

  two frequency polygons overlaidnext