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3.4.2 Validity of the Test
Validity is a standard or criterion that shows whether the instrument is valid or not. In this research, the validity of each item was calculated by using the product
moment formula:
In which, r
xy
: coefficient of correlation between x and y variable or validity of each item
N : the number of students participating in the test
Σx : the sum of score in each item
Σx
2
: the sum of the square score in each item Σy
: the sum of total score from each student Σy
2
: the sum of the square score from each student Σxy : the sum of multiple of score from each student with the total
score in each item
3.4.3 Reliability of the Test
Reliability shows whether an instrument is reliable and can be used as a device to collect the data. To measure the reliability of the test, the writer used used the
Kuder- Richardson Formula. The formula is as follows:
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In which, r
11
= the reliability of the test n
= the number of the items p
= the percentage of the students who answered the items correctly q
= the percentage of the students who answered the items incorrectly
S
2
= variance
3.4.4 Difficulty Level
After the try out was conducted and got the result, each item is classified into
difficulty level by using this formula:
T R
ID =
In which, ID = item difficulty
R = the number of the students who answered the item correctly T = the total number of the students
The level of difficulty of each item was determined by using these following categorizations:
Interval of difficulty level Criteria
0.00 ID 0.30
Difficult 0.30
ID 0.70 Medium
0.70 ID 1.00
Easy
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3.4.5 Discriminating Power
The discriminating power measures how well the test items arranged to identify
the differences in the students’ competence.
The formula is:
T RL
RU DP
2 1
− =
In which, DP
: discriminating power RU
: the number of the students in the upper group who answered the item correctly
RL : the number of students in the lower group who answered the item
correctly T
: the total number of the students
The criteria of the discrimination index are: Interval DP
Criteria 0.00 DP 0.20
Poor 0.20 DP 0.40
Satisfactory 0.40 DP 0.70
Good 0.70 DP 1.00
Excellent