When the class intervals are continuous but each of the given classes of unequal width, then we can find the mean with the help of any of the 3 methods :
Assumed mean method
Step deviation method
Consider the following example.
A class teacher has the following absentee record of 40 students of a class for the whole term. Find the mean number of days a student was absent.
|No. of days||0-6||6-10||10-14||14-20||20-28||28-38||38-40|
|No. of students||11||10||7||4||4||3||1|
Here is the solution.
|CLASS INTERVAL||FREQUENCY (fi)||CLASS MARKS(xi)||fixi|
|Total||∑ fi = 40||∑ fi xi = 499|
Mean = ∑ fi xi / ∑ fi = 499/40 = 12.475 ≈ 12.48
Note: Direct method should be used when the given data is small. Assumed mean method for intermediate data and for huge data we can use step deviation method.