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Educational Measurement and Evaluation

Notes students and ‘Low Marks’ students. Blood and Budd (1972) provides the following guidelines on
the meaning of the discrimination index as follows :
Value Description Suggestion
> 0.40 high discrimination Question is retained
0.20 – 0.39 moderate discrimination Question is checked & revised
0.10 – 0.19 low discrimination Question is removed or rewritten
0.00 & negative no discrimination Question is removed

Table 10.2 Interpretation of the Discrimination Index
A question that has a high discrimination index is able to differentiate between students who
know and do not know the answer. When we say that a question has a low discrimination index,
it is not able to differentiate between students who know and students who do not know. A low
discrimination index means that more “Low Marks’ students got the correct answer because the
question was too simple. It could also mean that students from both the ‘High Marks’ group and
‘Low Marks’ group got the answer wrong because the question was too difficult.
The formula for the discrimination index is such that if more students in the ‘High Marks’ group
chose the correct answer than did students in the low scoring group, the number will be positive.
At a minimum, then, one would hope for a positive value, as that would indicate that it is
knowledge of the question that resulted in the correct answer.
• The greater the positive value (the closer it is to 1.0), the stronger the relationship is between
overall test performance and performance on that item.
• It the discrimination index is negative, that means that for some reason students who scored
low on the test were more likely to get the answer correct. This is a strange situation which
suggests poor validity for an item.

What is Horst’s formula?

10.8 Calculating Discriminating Value by Formula

To calculate discriminating value of an item by the formula method, we arrange answer books in
a definite descending order after marking them. In this, the answer books with higher scores will
be at the top and those with lower scores will be at the bottom. Now, the top 1/3 of the answer
books are separated into one group and the bottom 1/3 of the answer books are segregated into
another group. The remaining answer books will not be needed. It is clear that the top 1/3
answer books belong to good students and the bottom 1/3 answer books to weak students. Now,
we have to calculate discrimination between these two groups. At first, it is found out what
percent of students in the top group have solved an item correctly, and how many have not.
These will be called P , Q respectively. Thus, P and Q are calculated for each item in the top
1 1 1 1
group. In the same way, the percentage of students in the bottom group solving each item
correctly is found out and how many of them have not solved them correctly. These two will be
called P and Q respectively. Here too, P and Q will be calculated for each group. Supposing,
1 2 2 2
the number of students in the top group is N and the number in the bottom group is N . Though
1 1
N and P are equal, because we take 1/3 of the total number of students.
1 2
Supposing, we know P , Q , P , Q , N and N for each item. Now, these values are applied in the
1 1 2 2 1 2
following formula in order to know their discriminating value for each item :

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