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.
|
Number 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 |
To find the class mark of each interval, the following relation is used.
Taking 17 as assumed mean (a), di and fidi are calculated as follows.
|
Number of days |
Number of students fi |
xi |
di = xi − 17 |
fidi |
|
0 − 6 |
11 |
3 |
− 14 |
− 154 |
|
6 − 10 |
10 |
8 |
− 9 |
− 90 |
|
10 − 14 |
7 |
12 |
− 5 |
− 35 |
|
14 − 20 |
4 |
17 |
0 |
0 |
|
20 − 28 |
4 |
24 |
7 |
28 |
|
28 − 38 |
3 |
33 |
16 |
48 |
|
38 − 40 |
1 |
39 |
22 |
22 |
|
Total |
40 |
− 181 |
From the table, we obtain
Therefore, the mean number of days is 12.48 days for which a student was absent.