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Educational Measurement and Evaluation

Notes 10. .50 + 3 Selected

11. .50 + 3 Selected
12. .80 + 2 Selected
13. .80 + 2 Selected
14. .60 0 Excluded
15. .40 – 2 Excluded
16. .70 + 1 Excluded
Thus, items 1, 2, 3, 5, 6, 8, 9, 10, 11, 13 and 13 have been selected for the final form of the test.

10.5.2 Calculating Difficulty Index By Formula
According to the formula method, the following formula is used to calculate difficulty level of
items :

N i
D.I. = N t × 100

Where, D.I. = Item difficulty Index
N = Number of students attempting it correctly
i
N = Total number of students.
t
For example, in a test, 120 out 150 students have correctly attempted an item, and 30 students
have attempted it incorrectly, then the difficulty index of that item will be :

120
D.I. = × 100
150
= 80% or .80

10.6 Correcting Difficulty Indices for Chance Errors

10.6.1 Guilford’s Formula
There is every possibility while administering a multiple items that students will guess answers
without actually knowing the right responses. In order to curb this tendency, the correction
formula is applied, which is are as follows :
⎛ W ⎞ 1
P = ⎜ R − ⎟
− ⎠
c ⎝ K1 N
Where, P = The percentage of those who know the right responses.
c
R = The percentage of those who give right response.
W = The percentage of those who give wrong response.
N = The number of candidates in the sample.
K = The number of alternatives.
Example : Supposing out of a total of 300 students, only 240 students have given the right
response for an item, and this item had four alternatives. Now, the difficulty level of this item
will be :

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