Calculation of the difficulty index of a question
Difficulty index Index
for measuring the easiness or difficulty of a test question. It is the
percentage (%)
of students who have
correctly answered a test question j it
would be more logical to call it the easiness index. It can vary from 0 to
100%.
Calculation
The
following formula is used:
Difficulty
index = [(H +L)
/ N] x 100
where
H = number
of correct answers in the
high group
L = number
of correct answers in
the
low group
N
= total number of students in both groups
Calculation of the discrimination index of a question
Discrimination
index
An indicator showing how significantly a question
discriminates between "high" and "low" students. It varies from 1 to +1.
Calculation
The following formula is used:
Discrimination index = 2
x [(H  L)
/N ]
Critical evaluation of a question
This is based on the indexes obtained.
Difficulty index: The higher this index the easier the question j it is thus an illogical term. It
is sometimes called "easiness index", but in the American literature
it is always
called "difficulty index". In principle, a
question with a difficulty index lying between 30% and 70%* is acceptable (in
that range, the discrimination index is more likely to be high).
If for a test you use a group of questions with indexes
in the range 30%  70%, then the mean index will be around
50%. It has been shown that a test with a difficulty index in the range of 50%
 60% is very likely to be reliable as regards its internal consistency or
homogeneity.
Discrimination index: The higher the index the more a question
will distinguish (for a given group of students) between "high" and
"low" students. When a test is composed of questions with high discrimination
indexes, it ensures a ranking that clearly discriminates between the students
according to their level of performance, i.e., it gives no advantage to the low
group over the high group. In other words, it helps you to find out who are
the best students.
It is most useful in preparing your question bank. Using
the index", you can judge questions as follows:
0.35 and over

Excellent question

0.25 to 0.34

Good question

0.15 to 0.24

Marginal question revise

under 0.15

Poor question – most likely
discard

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