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.
Numbers of days: | 0-6 | 6-10 | 10-14 | 14-20 | 20-28 | 28-38 | 38-40 |
Number of students: | 11 | 10 | 7 | 4 | 4 | 3 | 1 |
We may calculate class marks (xi) for each interval by using the relation
xi = 
Now taking assumed mean (A) = 16
Number of days | Number of students (fi) | xi | di = xi - 16 | fidi |
0-6 | 11 | 3 | -13 | -143 |
6-10 | 10 | 5 | -8 | -80 |
10-14 | 7 | 12 | -4 | -28 |
14-20 | 4 | 16 | 0 | 0 |
20-28 | 4 | 24 | 8 | 32 |
28-38 | 3 | 33 | 17 | 51 |
38-40 | 1 | 39 | 23 | 23 |
N = 40 | ∑fidi = -145 |
Mean (x̅) = A + 
= 16 + 
= 16 – 3.62 = 12.38
So, mean number of days is 12.38 days for which students were absent.
24