Elementary Statistics (1+1)

5.2.5.4. Ogives or  Cumulative Frequency curves

At times we are interested in knowing ‘how many workers of a factory earn less than Rs.700 per month’ or ‘how many workers earn more than Rs.1000 per month’ ‘percentage of students who have failed, etc. To answer these questions, it is necessary to add the frequencies. When frequencies are added, they are called cumulative frequencies. These frequencies are then listed in a table called a cumulative frequency table. The curve obtained by plotting cumulative frequencies is called a cumulative frequency curve or an Ogive (pronounced Ojive).

There are two methods of constructing Ogive, namely:

(a) The ‘less than’ method.

(b) The ‘more than’ method.

(a) ‘Less than’ method. In the ‘less than’ method we start with the upper limits of the classes and go on adding the frequencies are plotted we get a rising curve.

(b) ‘More than’ method. In the ‘more than’ method we start with the lower limits of the classes and from the frequencies we subtract the frequency of each class. When these frequencies are plotted we get a declining curve.