NAVY SCHOOL MANAGEMENT MANUAL

The formula p = Nc ÷ N results in a proportion or
decimal that becomes the index of item difficulty.
The larger the index, the easier the item. If the
item is answered correctly by everyone, the index
would be 1.00. If no one answered it correctly, the
index would be 0.00. For 150 answer sheets, where 100
answers were correct, the difficulty index would be as
follows:
P = 100 = .66
150
Based on the limits, this item would be considered
acceptable.
Sometimes a difficulty of 1.00 may be desirable. This
normally occurs in the area of safety where the goal is for
everyone to answer the item correctly.
Effectiveness of the alternatives is used for multiple-choice
test items
The multiple-choice test item is only as good as its
alternatives. If the incorrect alternatives are illogical,
not plausible or absurd, the student may be able to select
the correct response without knowing the material.
This index calculates the number of students selecting each
alternative within the high and low groups. The steps are
as follows:
After sorting the answer sheets from highest to
lowest, select the highest and lowest 27 percent of
the students.
Count the number of students in each group that
selected each alternative. For example:
Item 1 (a) (b) (c) (d) Total
High 27% 2 15 17 7 41
Low 27% 1 12 15 13 41
Alternative "a" may need to be improved. It is
ineffective as an alternative since it was selected by
only 3 of 82 students.
C-17
NAVEDTRA 135C