other classes. After scoring the result of the try out, I make an analysis to find out the validity, reliability, discriminating power, and level difficulty of the try out
item. All of them are used to decide which items are good and proper to be tested.
3.8.1.1 Validity of the Test
A test is considered to be a good test if it has validity. Validity is a standard or criterion that shows whether the instrument is valid or not. An instrument of the
study is a called valid if it measures what should be measured. A valid instrument has high validity. To calculate the validity of each item, I used the product
moment formula. The formula is as follows;
Where, r
xy
= coefficient of correlation between x and y variable or validity of each item N = the number of students or subject participating in the test.
x = the sum of score in each item x
2
= the sum of the square score in each item
y = the sum of total score from each student y
2
= the sum of the square score from each student
xy = the sum of multiple of score from each student with the total score
in each ieach item.
This formula was used for validating each score, and the result was consulted to critical value for r-product moment. When obtained coefficient of
2 2
2 2
xy
r
correlation is higher than the critical value for r-product moment, it means that a scoring is valid at 5 alpha level of significance.
3.8.1.2 Reliability
Heaton 1979:154 states, reliability is necessary characteristic of any good test: for it to be valid at all, a test must be reliable as a measuring instrument.
Reliability is a consistency of its measurement whether it was administered in different occasion. In this study, the reliability of the test was measured by
comparing the obtained value with research-value product moment. That is, if the obtained value is higher than the table r-value, it can be said that the test is
reliable. To calculate the reliability of the test, I used this formula: �
11
= �
� − 1 1
− �
� 2
�
� 2
where, �
11
= index reliablity
� = number of item �
� 2
= item variance
�
� 2
= total variance
Arikunto 2006: 196
3.8.1.3 The Difficulty of an Item
