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Educational Measurement and Evaluation

Notes the two groups who answered incorrectly. In other words, 54% of students selected the correct
answer. Although our computation is based on the high and low scoring groups only, it provides
a close approximation to the estimate that would be obtained with the total group. Thus, it is
proper to say that the index of difficulty for this item is 54% (for this particular group). Note that,
since difficulty refers to the percentage getting the item right, the smaller the percentage figure
the more difficult the item. Lein (1980) provides these guidelines on the meaning of the difficulty
index as follows :

> 0.70 = Easy
0.30 – 0.69 = Moderate
< 0.29 = Difficult

Table. 8.4 Interpretation of the Difficulty Index (p)
If a teacher believes that the achievement 0.54 on the item is too low, he or she can change the
way he or she teaches to better meet the objective represented by the item. Another interpretation
might be that the item was too difficult or confusing or invalid, in which case the teacher can
replace or modify the item, perhaps using information from the item’s discrimination index or
distractor analysis.
Activity
A teacher gave a 20 item science test to a group of 35 students. The correct answer for
Question # 25 is ‘C’ and the results are as follows :
Options A B C D Blank
High Marks Group (n = 10) 0 2 8 2 0
Low Marks Group (n – 10) 2 4 3 2 1
(a) Compute the difficulty index (p) for Questions # 25.
(b) Is Question # 25 an easy or difficult question ?
(c) Do you think you need to improve Question # 25 ?

10.5 Calculation of Difficulty Value of Items

The ratio of students’ group that solves a question correctly or knows it actually is called its
difficulty index. Difficulty index assists to arrange items in sequence, and indicates what item
will be placed first and what item will follow it or will come in the end. There is no definite
suitable method to ascertain difficulty index, still following are the chief methods used to calculate
difficulty index :

1. By right response of the 27% top group and 27% bottom group.
2. 35% Harper’s facility index method.
3. By right response of the 50% top group and 50% bottom group.
4. By right response of the 33% top group and 33% bottom group.
5. By formula method.
Generally, 27% top and 27% bottom group method is used, but 50% top and 50% bottom group
is taken in case of a smaller group (N < 100) Here we shall discuss only the first four methods
only :

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