6.4 Frequency Polygons

6.4 Frequency Polygons

1. A frequency polygon is a line graph that connects the midpoints of 
    each class interval at the top end of each rectangle in a histogram.

2. A frequency polygon can be drawn from a
(a) Histogram,
(b) Frequency table

3. Steps for drawing a frequency polygon:
Step 1: Add a class with 0 frequency before the first class and add also a 
             class with 0 frequency after the last class.
Step 2: Calculate the midpoints or mark the midpoints of the top sides of 
             the rectangular bars including the midpoints of the two 
             additional classes.
Step 3: Joint all the midpoints with straight lines.

Example:
The following frequency table shows the distance travelled by 38 teenagers by motorcycles in one afternoon.

Journey travelled (km)
Frequency
55 – 59
4
60 – 64
4
65 – 69
7
70 – 74
8
75 – 79
9
80 – 84
6

Draw a frequency polygon based on the frequency table.

Solution:

Journey travelled (km)
Frequency
Midpoint
50 – 54
0
52
55 – 59
4
57
60 – 64
4
62
65 – 69
7
67
70 – 74
8
72
75 – 79
9
77
80 – 84
6
82
85 – 89
0
87