a. Content Validity
Best and Kahn say that content validity refers to the degree to which the test actually measures or is specifically related to the traits for which it was design, content,
validity is based upon the careful examination of course textbooks, syllabus, objectives, and the judgments of subject matter specialists.
10
It means that content validity is the level validity of the instrument that can be taken from course
textbooks, syllabus, objectives, and the judgments of subject matter specialists. To get content validity, the test adapted with the textbook and based on the syllabus for
the Eighth grade of MTS Al-Hidayah Srikuncoro. See Appendix 25
b. Construct Validity
Best and Kahn say that construct validity is the degree to which scores on a test can be accounted for by explanatory construct of sound theory.
11
It means that construct validity is showing the measurement used contained correct operation definite, which
was based on the theoritical concept. To make sure, the researcher consulted to the Expert Mr. Iwan Kurniawan M.Pd. for determining whether the test has obtained
construct validity or not. The researcher did construct valiidity by consulting to the Expert Mr. Iwan
Kurniawan M.Pd on 9
th
October, 2015. After the Expert analyzed the instruments and he concerned that the instruments were valid and he said that it needed to give a
10
John W. Best and James V. Kahn, Research in Education 7th ed New Delhi, PrenticeHall, 1995, p. 219
11
Ibid,p.219
few minute for the validation of the istrument based on the content of vocabulary test. The result of validation for vocabulary test is suitable with the content of vocabulary
test and the instrument of the test had agreement. See Appendix 5
c. Internal validity
An instrument has internal validity if every single instrument supports the mission holistically. To know the validity, the researcher used point Biserial Correlation
formula as follows :
R
pbis
= M
p
–M
t
=
Where :
R
pbis
= Coefisient of validity item M
p
= The average score of the right answer M
t
=The average of total score SD
t
=Standar Deviation P
=Proportional of the students who get true answer Q
= Proportional of the students who get wrong answer.
12
To get internal validity the researcher used Point Biserial correlation formula. In using this formula the researcher did until 3 steps. The aim was to find the valid of
the test items that can be used to collect the data of the research. In the first step of pre-test try out, there were 17 items considered invalid. They were the items number
2, 3, 4, 8, 10, 12, 16, 19, 20, 21, 22, 23, 24, 27, 29, 31, dan 36. After dropping those
12
Anas Sudijono, Pengantar Statistik Pendidikan. jakarta: Rajawali pers. 2012., p.258.
SD
t
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