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Y2
Based on the table above, two classes were selected as the experimental group and the control group. E is the experimental group that was given X (treatment) and C is the control group that is not given treatment. the pre-test (Y1) was given to the control group and the experimental group, but in the experimental group treatment would be given after the pre-test. Post-tests (Y2) is held to assess students' writing descriptive text ability.
Population and Sample
Population as defined by Ary et al (2010:148) is all members of any well-defi ned class of people, events, or objects. It means that a population  consists of various kinds of differences that come together in a class or scope.
In this research, the population is the first grade students of Pelita Bunga Bangsa Vocational School. There are 3 classes of first grade that consists of Electrical Engineering class = 22 students, Motorcycle Engineering class = 25 students, and Banking and Microfinance class = 33 students. So the population are 80 students in three classes.
Research sample is a part of the population that become the object of study. Sample according to Ary et al (2010:148) is a portion of a population. The sample of this research is only two classes of first grade students. The writer uses randomly sampling to determine sample. This is based on assumption that random selection will give fairer chance for the respondents to be chosen as the sample of this research. The result of cluster random sampling is Electrical Engineering and Motorcycle Engineering class = 22 students. The writer determines Electrical Engineering class as experimental group and Motorcycle Engineering class as control group. Electrical Engineering consist of 22 students and Motorcycle Engineering consist of 25 students. The writer took 22 students for each class. So the total of sample is 44 students.
Research Instrument
In collecting the data, several instrument are employed in this research, those are tests (pre-test and post-test) and questionnaire which are administered to both groups (experimental and control groups), differentiated by their administering time as pre-test is given before the treatment and post-testis given after the treatment. While questionnaire is given in the end of the research and only administered to the experimental group.
In this research, pre-test and post-test are the instruments to answer the research question about the use of Picture Series as Teaching Media to improve students' Descriptive Text Writing Skill. While questionnaire is used to investigate the responses of the students toward the use of Picture Series as Teaching Media to improve students' Descriptive Text Writing Skill.
Pre-test is defined as an activity to test the level of students' knowledge of the material to be delivered, pre-test activities carried out before teaching activities are given. Pre-test is given to all students, both Electrical Engineering as experimental group and Motorcycle Engineering as control group. Pre-test is given to know whether the experimental and control groups are in the same level at beginning or not. The form of pre-test is a completion, a random short paragraph and a description text writing test. So, the total of pre-test is 3 items.
