26
Each test item in this study especially its content was designed based on the material had taught in order to be valid. To get the validity of each item, the writer
analyzed statistically. The writer used the γ
pbis
formula as follows:
In which: γ
pbis
: Coefficient Biseral correlation M
p
: Average of students who answered item no.1 correctly. M
t :
Average of total score S
t
: Standard deviation of total score p
: number of students who answered item no.1 correctly. q
: number of students who answered item no.1 incorrectly. Criteria:
γ
pbis
r
table,
so the item is valid
3.7.4 Reliability
Reliability was a general quality of stability of scores regardless of what the test measured.
In this study, the writer decided to use K-R.20 in measuring reliability of the test. This formula uses the number of items and the test, the mean of the scores,
and the square of the deviation. Consulting the K-R. 20, the computation of the reliability of the test used the
following formula:
q p
S M
M γ
t t
p pbis
− =
27
In which:
r
11
: Reliability of total test. p
: number of students who answered item no.1 correctly. q
: number of students who answered item no.1 incorrectly. ∑pq : The sum of the result p x q
n : number of students
s : Standard deviation of the test.
3.7.5 Item Analysis
After determining and scoring the try-out test, an item analysis was made to evaluate the effectiveness of the items. It was meant to check whether each item
met the requirement of a good test item. This item analysis concentrated two vital features, level of difficulty and discriminating power.
Heaton 1975: 172 said that all item should be examination from the point of view of their difficulty level of discrimination.
3.7.6 Index of Difficulty
The index of difficulty level is generally expressed as the percentage of the students who answered the item correctly. It can be calculated by using the
formula according to Arikunto 2006: 178 as follows:
⎟⎟⎠ ⎞
⎜⎜⎝ ⎛
∑ −
⎟ ⎠
⎞ ⎜
⎝ ⎛
=
2 2
11
S pq
S 1
- k
k r
28
P=
JS B
Where: P : index of difficulty
B : number of the students who answered item correctly. JS : number of the students in a class.
Criteria
ID interval Criteria
0.00 ID ≤ 0.30
0.30 ID ≤ 0.70
0.70 ID 1.00 difficult
medium easy
3.7.7 Discriminating Power
The discriminating power is a measure of the effectiveness of an item discriminating between high and low scores of the whole test. The higher the
values of discriminating power are, the more effective the item will be. Heaton 1975:173 states the discrimination index of an item indicated the
extent to which the item discriminated between the tested, separating the more able tested from the less able.
The index of discriminating told us whether those students who performed well on the whole test tended to do well or badly on each item in the test.
Discriminating power can be obtained by using this following formula:
29
Where: DP : Discriminating Power
JB
A
: number of student in upper group who answered the item correctly JB
B
: number of student in lower group who answered the item incorrectly JS
A
: number of students in upper group Criteria
DP interval Criteria
0.00 DP ≤ 0.20
0.20 DP ≤ 0.40
0.40 DP ≤ 0.70
0.70 DP ≤ 1.00
poor satisfactory
good excellent
3.8 The Pre Test
