Page 5 - Lesson note-4- Ch 14 Statistics( Frequency Polygon)

Example4:
Draw a frequency polygon for the following frequency distribution:

Class Frequency
interval
1-10 8
11-20 3
21-30 6
31-40 12
41-50 2
51-60 7
Solution:

Convert the given inclusive form into an exclusive form by taking intervals as 0.5 – 1.5,
10.5 – 20.5, 20.5 – 30.5, 40.5 – 40.5, 40.5 – 50.5 and 50.5 – 60.5
We know that
Class mark of a class interval = (lower limit + upper limit)/ 2

Class interval True class limit Class mark Frequency
(-9) -0 (-9.5) - 0.5 -4.5 0
1 -10 0.5 - 10.5 5.5 8
11 -20 10.5-20.5 15.5 3
21 -30 20.5-30.5 25.5 6
31 -40 30.5-40.5 35.5 12
41 -50 40.5-50.5 45.5 2
51 -60 50.5-60.5 55.5 7
61 -70 60.5-70.5 65.5 0
Consider the class intervals that is marks on the X – axis and frequency on the Y- axis.
Plot the points and join them to obtain the frequency polygon.