Page 308 - DEDU504_EDUCATIONAL_MEASUREMENT_AND_EVALUATION_ENGLISH

Educational Measurement and Evaluation

Notes DP = PQ
Where, DP = Discriminating Power
P = The percentage of students solving an item correctly
Q = The percentage of students solving an item incorrectly
From the form of the above formula, it becomes evident that if an item is solved correctly
by all students or by none of them, the value of Q and P becomes zero respectively. In
such a condition, the discriminating power will be zero. The specialists are of the view
that the item with discriminating power 250 is considered suitable for the test.
Example
The item number 7 of an achievement test was solved correctly by 30 students and
incorrectly by 20 students in a class of 50 students. Find out the discriminating power of
this item.
Solution
The number of students solving the item correctly = 30
Percentage of students solving the item correctly

30
P= ×100
50
= 60%
And percentage of students solving the item incorrectly
Q = 100 – P
= 100 – 60
= 40%
So, discriminating power
DP = PQ
×
= 60 40
= 2400
Second Method : When the number of students in a group is large, they are arranged in
descending order according to the scores obtained in the test; and after this, two groups
are formed as ‘high’ and ‘low’. Different methods for forming these two groups are used
which are shown in the following table :

Number of Students
Class
I II III IV

High 25% 27% 33% 50%
Low 25% 27% 33% 50%

According to Geret and some other psychologists, the discriminating power can be
better obtained by grouping 27% students in the high group and 27% in the low group.
However, in research work where the number of students is 100 or less, the classes of 50-
50% give better results.
In the method of ‘high’ and ‘low’ classes, the discriminating power is found out by using
the following formula :

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