3.5.2.1 Validity of the Test
Validity is a measurement which shows the grades of number of an instrument. A valid instrument must have high validity, it means that an
instrument which lacks validity is said to be invalid instrument Suharsimi, 2002:144.
And later suharsimi said that an instrument is called a valid one when it can measure something which is wanted by uncovering the variable studied
exactly. The method used in measuring the validation of the instrument is called content validity. Suharsimi also said that a test or a measurement can be called a
content test when it measures the special purpose which is equal with the material or content given.
The method used to measure the validity of the test is calculated with the formula of Pearson
= the square of the sum of vocabulary test scores N
= the total number of the subjects If rxy r table, it means that the test is valid with the significant degree 5.
The validity computation consulted with the r-table of Product Moment, therefore the correlation could be known whether it is significant or not. If r-table
value is less the value of critical table, the correlation is not significant. 1
Item validity for vocabulary mastery For example to calculate validity for vocabulary mastery, the writer used item
number 2. From appendixes of table for validity, reliability, discriminating power, and difficulty item for vocabulary mastery showed that:
N = 30, = 152100
From the computation above, the formula got the validity of t-test number 1 is 0, 61. Then she consulted to the standard validity. Since
was between 0, 60
From the computation above, the writer got the validity of t-test number 1 is
0, 50.
Then he
consulted to
the standard
validity. Since
was between 0, 49
Then
In this computation, the reliability coefficient of the vocabulary test is 0, 80. Then the writer consulted the standard reliability, since
was between 0, 80
To clearer the reliability of reading comprehension ability, we can see in appendixes. From the appendix reliability of reading comprehension, it shows
that N = 30,
Then
In this computation, the reliability coefficient of the reading comprehension is 0, 66. Then the writer consulted the standard reliability, since
was between 0, 60
From the computation, the level of difficulty of the vocabulary test number 3 was 0, 80. It means that the vocabulary test number 3 was easy, since it was
between 0, 70
3.5.2.4 Discriminating Power of an Item
Kategori